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/map/src/index.ts

@@ -1,5 +1,6 @@
-/**
- * Map rendering utilities for trails.cool
- * Leaflet wrappers, tile layer configs, overlay management
- */
-export {};
+export { MapView } from "./MapView";
+export type { MapViewProps } from "./MapView";
+export { RouteLayer } from "./RouteLayer";
+export type { RouteLayerProps } from "./RouteLayer";
+export { baseLayers } from "./layers";
+export type { TileLayerConfig } from "./layers";