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/MapView.tsx

@@ -0,0 +1,34 @@
+import { MapContainer, TileLayer, LayersControl } from "react-leaflet";
+import { baseLayers } from "./layers";
+import "leaflet/dist/leaflet.css";
+
+export interface MapViewProps {
+  center?: [number, number];
+  zoom?: number;
+  className?: string;
+  children?: React.ReactNode;
+}
+
+export function MapView({
+  center = [50.1, 10.0],
+  zoom = 6,
+  className = "h-full w-full",
+  children,
+}: MapViewProps) {
+  return (
+    <MapContainer center={center} zoom={zoom} className={className}>
+      <LayersControl position="topright">
+        {baseLayers.map((layer, i) => (
+          <LayersControl.BaseLayer key={layer.name} checked={i === 0} name={layer.name}>
+            <TileLayer
+              url={layer.url}
+              attribution={layer.attribution}
+              maxZoom={layer.maxZoom}
+            />
+          </LayersControl.BaseLayer>
+        ))}
+      </LayersControl>
+      {children}
+    </MapContainer>
+  );
+}