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/models/tile_provider.dart

@@ -1,6 +1,8 @@
 import 'dart:convert';
 import 'dart:typed_data';
 
+import '../services/service_policy.dart';
+
 /// A specific tile type within a provider
 class TileType {
   final String id;
@@ -10,7 +12,7 @@ class TileType {
   final Uint8List? previewTile; // Single tile image data for preview
   final int maxZoom; // Maximum zoom level for this tile type
 
-  const TileType({
+  TileType({
     required this.id,
     required this.name,
     required this.urlTemplate,
@@ -76,6 +78,15 @@ class TileType {
   /// Check if this tile type needs an API key
   bool get requiresApiKey => urlTemplate.contains('{api_key}');
 
+  /// The service policy that applies to this tile type's server.
+  /// Cached because [urlTemplate] is immutable.
+  late final ServicePolicy servicePolicy =
+      ServicePolicyResolver.resolve(urlTemplate);
+
+  /// Whether this tile server's usage policy permits offline/bulk downloading.
+  /// Resolved via [ServicePolicyResolver] from the URL template.
+  bool get allowsOfflineDownload => servicePolicy.allowsOfflineDownload;
+
   Map<String, dynamic> toJson() => {
     'id': id,
     'name': name,