Add offline-first tile system with per-provider caching and error retry

- Add ServicePolicy framework with OSM-specific rate limiting and TTL
- Add per-provider disk tile cache (ProviderTileCacheStore) with O(1)
  lookup, oldest-modified eviction, and ETag/304 revalidation
- Rewrite DeflockTileProvider with two paths: common (NetworkTileProvider)
  and offline-first (disk cache -> local tiles -> network with caching)
- Add zoom-aware offline routing so tiles outside offline area zoom ranges
  use the efficient common path instead of the overhead-heavy offline path
- Fix HTTP client lifecycle: dispose() is now a no-op for flutter_map
  widget recycling; shutdown() handles permanent teardown
- Add TileLayerManager with exponential backoff retry (2s->60s cap),
  provider switch detection, and backoff reset
- Guard null provider/tileType in download dialog with localized error
- Fix Nominatim cache key to use normalized viewbox values
- Comprehensive test coverage (1800+ lines across 6 test files)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/main.dart

@@ -14,6 +14,7 @@ import 'screens/release_notes_screen.dart';
 import 'screens/osm_account_screen.dart';
 import 'screens/upload_queue_screen.dart';
 import 'services/localization_service.dart';
+import 'services/provider_tile_cache_manager.dart';
 import 'services/version_service.dart';
 import 'services/deep_link_service.dart';
 
@@ -21,13 +22,16 @@ import 'services/deep_link_service.dart';
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
-  
+
   // Initialize version service
   await VersionService().init();
-  
+
   // Initialize localization service
   await LocalizationService.instance.init();
 
+  // Resolve platform cache directory for per-provider tile caching
+  await ProviderTileCacheManager.init();
+
   // Initialize deep link service
   await DeepLinkService().init();
   DeepLinkService().setNavigatorKey(_navigatorKey);