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>

.gitignore

@@ -73,12 +73,13 @@ fuchsia/build/
 web/build/
 
 # ───────────────────────────────
-# IDE / Editor Settings
+# IDE / Editor / AI Tool Settings
 # ───────────────────────────────
 .idea/
 .idea/**/workspace.xml
 .idea/**/tasks.xml
 .vscode/
+.claude/settings.local.json
 # Swap files
 *.swp
 *.swo