Implement shared packages (tasks 2.1-2.6)

- @trails-cool/types: Route, Activity, Waypoint, RouteVersion, RouteMetadata
- @trails-cool/gpx: GPX parser (XML→waypoints/tracks/elevation) and generator
  with round-trip test, haversine distance, elevation gain/loss computation
- @trails-cool/map: MapView (Leaflet + OSM/OpenTopoMap/CyclOSM layer switcher),
  RouteLayer (GeoJSON polyline rendering)
- @trails-cool/ui: Button (primary/secondary/ghost), Input (with label/error),
  Card components with Tailwind styling
- @trails-cool/i18n: react-i18next config with English + German translations,
  browser language detection, fallback to English

All 13 unit tests pass. Both apps build successfully.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/i18n/package.json

@@ -2,6 +2,15 @@
   "name": "@trails-cool/i18n",
   "version": "0.0.1",
   "type": "module",
+  "exports": {
+    ".": "./src/index.ts",
+    "./locales/*": "./src/locales/*"
+  },
   "main": "./src/index.ts",
-  "types": "./src/index.ts"
+  "types": "./src/index.ts",
+  "peerDependencies": {
+    "react": ">=18",
+    "i18next": ">=23",
+    "react-i18next": ">=14"
+  }
 }