Initial commit: Mathe-App Phase 1-3

- React+Vite Frontend mit Routing, eigenem fetch-Client (kein axios)
- Express Backend: Auth (JWT), Topics, Tasks, Leaderboard
- PostgreSQL Schema + Seed: 7 Kategorien, 21 Topics, ~25 Aufgaben
- Gamification: XP, Level (100×n^1.5), tägliche Streaks
- docker-compose auf Port 3100 für DS1621
- Alltagsaufgaben: Finanzen, Geometrie, Physik, Informatik, Verkehr, Shopping

frontend/src/api/client.js

@@ -0,0 +1,41 @@
+const BASE = '/api'
+
+function getToken() {
+  return localStorage.getItem('token')
+}
+
+function headers(extra = {}) {
+  const h = { 'Content-Type': 'application/json', ...extra }
+  const token = getToken()
+  if (token) h['Authorization'] = `Bearer ${token}`
+  return h
+}
+
+async function request(method, path, body) {
+  const res = await fetch(`${BASE}${path}`, {
+    method,
+    headers: headers(),
+    credentials: 'include',
+    body: body !== undefined ? JSON.stringify(body) : undefined,
+  })
+
+  const data = await res.json().catch(() => ({}))
+
+  if (!res.ok) {
+    const err = new Error(data.message || `HTTP ${res.status}`)
+    err.status = res.status
+    err.data = data
+    throw err
+  }
+
+  return data
+}
+
+const api = {
+  get:    (path)        => request('GET',    path),
+  post:   (path, body)  => request('POST',   path, body),
+  put:    (path, body)  => request('PUT',    path, body),
+  delete: (path)        => request('DELETE', path),
+}
+
+export default api