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/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);