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(
-        data: (complete) => const AppShell(),
+      home: modelPresentAsync.when(
+        data: (present) =>
+            present ? const AppShell() : const ModelDownloadScreen(),
         loading: () => const Scaffold(
           body: Center(child: CircularProgressIndicator()),
         ),
-        error: (e, _) => const AppShell(),
+        error: (_, __) => const ModelDownloadScreen(),
       ),
     );
   }