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>

package.json

@@ -27,15 +27,21 @@
     "@tailwindcss/vite": "^4.2.2",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.2",
+    "@types/leaflet": "^1.9.21",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "eslint": "^10.1.0",
     "eslint-config-prettier": "^10.1.8",
+    "i18next": "^25.10.4",
+    "i18next-browser-languagedetector": "^8.2.1",
     "jsdom": "^29.0.1",
+    "leaflet": "^1.9.4",
     "playwright": "^1.58.2",
     "prettier": "^3.8.1",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
+    "react-i18next": "^16.6.1",
+    "react-leaflet": "^5.0.0",
     "react-router": "^7.13.1",
     "tailwindcss": "^4.2.2",
     "turbo": "^2.8.20",