feat: add model download screen and gate app on model presence

Adds a one-time download flow for the on-device Gemma 2B model (~1.1 GB)
with a progress bar, partial-download cleanup, and automatic navigation
to the app shell once the file is verified. App.dart now gates on
modelPresentProvider instead of onboarding state.

lib/app.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
-import 'core/providers/settings_provider.dart';
+import 'core/providers/model_download_provider.dart';
+import 'features/model_download/model_download_screen.dart';
 import 'shell.dart';
 
 class App extends ConsumerWidget {
@@ -8,7 +9,7 @@ class App extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
-    final onboardingAsync = ref.watch(isOnboardingCompleteProvider);
+    final modelPresentAsync = ref.watch(modelPresentProvider);
 
     return MaterialApp(
       title: 'DailyYou',
@@ -16,12 +17,13 @@ class App extends ConsumerWidget {
       theme: _lightTheme(),
       darkTheme: _darkTheme(),
       themeMode: ThemeMode.system,
-      home: onboardingAsync.when(
+      home: modelPresentAsync.when(
-        data: (complete) => const AppShell(),
+        data: (present) =>
+            present ? const AppShell() : const ModelDownloadScreen(),
         loading: () => const Scaffold(
           body: Center(child: CircularProgressIndicator()),
         ),
-        error: (e, _) => const AppShell(),
+        error: (_, __) => const ModelDownloadScreen(),
       ),
     );
   }

lib/core/llm/model_download_service.dart

@@ -0,0 +1,63 @@
+import 'dart:io';
+import 'package:path_provider/path_provider.dart';
+import 'package:path/path.dart' as p;
+
+class ModelDownloadService {
+  static const modelFileName = 'gemma-2b-it-cpu-int4.bin';
+  static const modelSizeBytes = 1183285248; // ~1.1 GB
+
+  // Gemma 2B IT CPU int4 — MediaPipe LLM Inference model
+  static const _downloadUrl =
+      'https://storage.googleapis.com/mediapipe-models/llm_inference/'
+      'gemma-2b-it-cpu-int4/float32/1/gemma-2b-it-cpu-int4.bin';
+
+  Future<String> get modelPath async {
+    final dir = await getApplicationSupportDirectory();
+    return p.join(dir.path, modelFileName);
+  }
+
+  Future<bool> isModelPresent() async {
+    final path = await modelPath;
+    final file = File(path);
+    if (!file.existsSync()) return false;
+    // Verify the file isn't a partial download (allow small variance)
+    final size = await file.length();
+    return size > modelSizeBytes * 0.99;
+  }
+
+  /// Streams download progress as a value from 0.0 to 1.0.
+  Stream<double> download() async* {
+    final path = await modelPath;
+    final file = File(path);
+    final client = HttpClient();
+
+    try {
+      final request = await client.getUrl(Uri.parse(_downloadUrl));
+      final response = await request.close();
+
+      if (response.statusCode != 200) {
+        throw Exception('Download failed: HTTP ${response.statusCode}');
+      }
+
+      final totalBytes =
+          response.contentLength > 0 ? response.contentLength : modelSizeBytes;
+      int receivedBytes = 0;
+
+      final sink = file.openWrite();
+      try {
+        await for (final chunk in response) {
+          sink.add(chunk);
+          receivedBytes += chunk.length;
+          yield (receivedBytes / totalBytes).clamp(0.0, 1.0);
+        }
+      } finally {
+        await sink.close();
+      }
+    } catch (_) {
+      if (await file.exists()) await file.delete();
+      rethrow;
+    } finally {
+      client.close();
+    }
+  }
+}

lib/core/providers/model_download_provider.dart

@@ -0,0 +1,38 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import '../llm/model_download_service.dart';
+import 'settings_provider.dart';
+
+// ── Is the model file present on disk? ───────────────────────────────────────
+
+final modelPresentProvider = FutureProvider<bool>((ref) {
+  return ModelDownloadService().isModelPresent();
+});
+
+// ── Download notifier — streams progress and marks completion ─────────────────
+
+class ModelDownloadNotifier extends Notifier<AsyncValue<double>> {
+  @override
+  AsyncValue<double> build() => const AsyncData(0.0);
+
+  Future<void> startDownload() async {
+    state = const AsyncData(0.0);
+    try {
+      await for (final progress in ModelDownloadService().download()) {
+        if (!ref.mounted) return;
+        state = AsyncData(progress);
+      }
+      if (!ref.mounted) return;
+      await ref.read(settingsNotifierProvider.notifier).markModelDownloaded();
+      ref.invalidate(modelPresentProvider);
+    } catch (e, st) {
+      if (!ref.mounted) return;
+      state = AsyncError(e, st);
+    }
+  }
+
+  void reset() => state = const AsyncData(0.0);
+}
+
+final modelDownloadNotifierProvider =
+    NotifierProvider<ModelDownloadNotifier, AsyncValue<double>>(
+        ModelDownloadNotifier.new);

lib/features/model_download/model_download_screen.dart

@@ -0,0 +1,167 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import '../../core/providers/model_download_provider.dart';
+
+class ModelDownloadScreen extends ConsumerWidget {
+  const ModelDownloadScreen({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final downloadState = ref.watch(modelDownloadNotifierProvider);
+
+    return Scaffold(
+      body: SafeArea(
+        child: Padding(
+          padding: const EdgeInsets.all(32),
+          child: downloadState.when(
+            data: (progress) => progress == 0.0
+                ? _IdleView(
+                    onDownload: () => ref
+                        .read(modelDownloadNotifierProvider.notifier)
+                        .startDownload(),
+                  )
+                : _DownloadingView(progress: progress),
+            loading: () => const Center(child: CircularProgressIndicator()),
+            error: (e, _) => _ErrorView(
+              error: e.toString(),
+              onRetry: () {
+                ref.read(modelDownloadNotifierProvider.notifier).reset();
+              },
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+class _IdleView extends StatelessWidget {
+  const _IdleView({required this.onDownload});
+
+  final VoidCallback onDownload;
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      mainAxisAlignment: MainAxisAlignment.center,
+      crossAxisAlignment: CrossAxisAlignment.stretch,
+      children: [
+        Icon(
+          Icons.auto_awesome,
+          size: 64,
+          color: Theme.of(context).colorScheme.primary,
+        ),
+        const SizedBox(height: 24),
+        Text(
+          'On-device AI',
+          style: Theme.of(context).textTheme.headlineSmall,
+          textAlign: TextAlign.center,
+        ),
+        const SizedBox(height: 12),
+        Text(
+          'DailyYou uses an on-device AI model to spot patterns in your moods '
+          'and behaviour. Your data never leaves your phone.',
+          style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+                color: Theme.of(context).colorScheme.onSurfaceVariant,
+              ),
+          textAlign: TextAlign.center,
+        ),
+        const SizedBox(height: 8),
+        Text(
+          'The model is ~1.1 GB and only needs to be downloaded once.',
+          style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                color: Theme.of(context).colorScheme.outline,
+              ),
+          textAlign: TextAlign.center,
+        ),
+        const SizedBox(height: 40),
+        FilledButton.icon(
+          onPressed: onDownload,
+          icon: const Icon(Icons.download),
+          label: const Text('Download model'),
+        ),
+      ],
+    );
+  }
+}
+
+class _DownloadingView extends StatelessWidget {
+  const _DownloadingView({required this.progress});
+
+  final double progress;
+
+  @override
+  Widget build(BuildContext context) {
+    final pct = (progress * 100).toStringAsFixed(0);
+    final done = progress >= 1.0;
+
+    return Column(
+      mainAxisAlignment: MainAxisAlignment.center,
+      crossAxisAlignment: CrossAxisAlignment.stretch,
+      children: [
+        Icon(
+          done ? Icons.check_circle : Icons.downloading,
+          size: 64,
+          color: done
+              ? Theme.of(context).colorScheme.primary
+              : Theme.of(context).colorScheme.secondary,
+        ),
+        const SizedBox(height: 24),
+        Text(
+          done ? 'Ready!' : 'Downloading…',
+          style: Theme.of(context).textTheme.headlineSmall,
+          textAlign: TextAlign.center,
+        ),
+        const SizedBox(height: 24),
+        LinearProgressIndicator(value: progress),
+        const SizedBox(height: 8),
+        Text(
+          done ? 'Model downloaded successfully.' : '$pct% — keep the app open',
+          style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                color: Theme.of(context).colorScheme.outline,
+              ),
+          textAlign: TextAlign.center,
+        ),
+      ],
+    );
+  }
+}
+
+class _ErrorView extends StatelessWidget {
+  const _ErrorView({required this.error, required this.onRetry});
+
+  final String error;
+  final VoidCallback onRetry;
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      mainAxisAlignment: MainAxisAlignment.center,
+      crossAxisAlignment: CrossAxisAlignment.stretch,
+      children: [
+        Icon(Icons.error_outline,
+            size: 64, color: Theme.of(context).colorScheme.error),
+        const SizedBox(height: 24),
+        Text(
+          'Download failed',
+          style: Theme.of(context).textTheme.headlineSmall,
+          textAlign: TextAlign.center,
+        ),
+        const SizedBox(height: 8),
+        Text(
+          'Check your connection and try again.',
+          style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+                color: Theme.of(context).colorScheme.onSurfaceVariant,
+              ),
+          textAlign: TextAlign.center,
+        ),
+        const SizedBox(height: 40),
+        FilledButton.icon(
+          onPressed: onRetry,
+          icon: const Icon(Icons.refresh),
+          label: const Text('Try again'),
+        ),
+      ],
+    );
+  }
+}