feat: content repository merges cached seasons with bundled fallback

Extends ContentRepository with optional cacheDir: cached packs in
<cacheDir>/seasons/*/pack.json merge with bundled ones (cached wins
for same id), corrupt/future-schema packs silently ignored, refresh()
fires ContentDownloader.sync() once per session. main() wires the real
cache+downloader instance; default constructor stays bundled-only for tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/main.dart

@@ -1,17 +1,48 @@
+import 'dart:io';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:path_provider/path_provider.dart';
 
 import 'app.dart';
+import 'data/content_repository.dart';
+import 'data/remote/content_downloader.dart';
 import 'data/save_repository.dart';
 import 'state/providers.dart';
 
+/// Remote content origin. Swap per environment with
+/// --dart-define=CONTENT_BASE_URL=...; the default points at the production
+/// Firebase Hosting site (owner setup pending).
+const contentBaseUrl = String.fromEnvironment(
+  'CONTENT_BASE_URL',
+  defaultValue: 'https://block-seasons.web.app/content',
+);
+
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
   final saveRepository = await SaveRepository.open();
-  runApp(
-    ProviderScope(
-      overrides: [saveRepositoryProvider.overrideWithValue(saveRepository)],
-      child: const BlockSeasonsApp(),
-    ),
-  );
+
+  ContentRepository contentRepository;
+  try {
+    final support = await getApplicationSupportDirectory();
+    final cacheDir = Directory('${support.path}/content');
+    contentRepository = ContentRepository(
+      cacheDir: cacheDir,
+      downloader: ContentDownloader(
+        baseUrl: contentBaseUrl,
+        cacheDir: cacheDir,
+      ),
+    );
+  } catch (e) {
+    debugPrint('content cache unavailable, bundled only: $e');
+    contentRepository = ContentRepository();
+  }
+
+  runApp(ProviderScope(
+    overrides: [
+      saveRepositoryProvider.overrideWithValue(saveRepository),
+      contentRepositoryProvider.overrideWithValue(contentRepository),
+    ],
+    child: const BlockSeasonsApp(),
+  ));
 }