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/ui/src/Input.tsx

@@ -0,0 +1,25 @@
+import type { InputHTMLAttributes } from "react";
+
+export interface InputProps extends InputHTMLAttributes<HTMLInputElement> {
+  label?: string;
+  error?: string;
+}
+
+export function Input({ label, error, className = "", id, ...props }: InputProps) {
+  const inputId = id ?? label?.toLowerCase().replace(/\s+/g, "-");
+  return (
+    <div className="flex flex-col gap-1.5">
+      {label && (
+        <label htmlFor={inputId} className="text-sm font-medium text-gray-700">
+          {label}
+        </label>
+      )}
+      <input
+        id={inputId}
+        className={`rounded-md border border-gray-300 px-3 py-2 text-sm shadow-sm transition-colors focus:border-blue-500 focus:outline-none focus:ring-1 focus:ring-blue-500 ${error ? "border-red-500" : ""} ${className}`}
+        {...props}
+      />
+      {error && <p className="text-sm text-red-600">{error}</p>}
+    </div>
+  );
+}