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/localizations/nl.json

@@ -181,7 +181,10 @@
     "offlineModeWarning": "Downloads uitgeschakeld in offline modus. Schakel offline modus uit om nieuwe gebieden te downloaden.",
     "areaTooBigMessage": "Zoom in tot ten minste niveau {} om offline gebieden te downloaden. Grote gebied downloads kunnen ervoor zorgen dat de app niet meer reageert.",
     "downloadStarted": "Download gestart! Tiles en nodes ophalen...",
-    "downloadFailed": "Download starten mislukt: {}"
+    "downloadFailed": "Download starten mislukt: {}",
+    "offlineNotPermitted": "De {}-server staat geen offline downloads toe. Schakel over naar een tegelserver die offline gebruik toestaat (bijv. Bing Maps, Mapbox of een zelf gehoste tegelserver).",
+    "currentTileProvider": "huidige tegel",
+    "noTileProviderSelected": "Geen tegelprovider geselecteerd. Selecteer een kaartstijl voordat u een offlinegebied downloadt."
   },
   "downloadStarted": {
     "title": "Download Gestart",
@@ -335,7 +338,10 @@
   "mapTiles": {
     "title": "Kaart Tiles",
     "manageProviders": "Beheer Providers",
-    "attribution": "Kaart Attributie"
+    "attribution": "Kaart Attributie",
+    "mapAttribution": "Kaartbron: {}",
+    "couldNotOpenLink": "Kon link niet openen",
+    "openLicense": "Open licentie: {}"
   },
   "profileEditor": {
     "viewProfile": "Bekijk Profiel",
@@ -552,4 +558,4 @@
     "metricDescription": "Metrisch (km, m)",
     "imperialDescription": "Imperiaal (mijl, ft)"
   }
-}
+}