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/Button.tsx

@@ -0,0 +1,32 @@
+import type { ButtonHTMLAttributes } from "react";
+
+export interface ButtonProps extends ButtonHTMLAttributes<HTMLButtonElement> {
+  variant?: "primary" | "secondary" | "ghost";
+  size?: "sm" | "md" | "lg";
+}
+
+const variantStyles = {
+  primary: "bg-blue-600 text-white hover:bg-blue-700 active:bg-blue-800",
+  secondary: "bg-gray-100 text-gray-900 hover:bg-gray-200 active:bg-gray-300",
+  ghost: "text-gray-700 hover:bg-gray-100 active:bg-gray-200",
+};
+
+const sizeStyles = {
+  sm: "px-2.5 py-1.5 text-sm",
+  md: "px-4 py-2 text-sm",
+  lg: "px-6 py-3 text-base",
+};
+
+export function Button({
+  variant = "primary",
+  size = "md",
+  className = "",
+  ...props
+}: ButtonProps) {
+  return (
+    <button
+      className={`inline-flex items-center justify-center rounded-md font-medium transition-colors focus:outline-none focus:ring-2 focus:ring-blue-500 focus:ring-offset-2 disabled:opacity-50 disabled:pointer-events-none ${variantStyles[variant]} ${sizeStyles[size]} ${className}`}
+      {...props}
+    />
+  );
+}