feat: stages module, mobility frontend, theme toggle, employeeOnly access control

- Add stages module with full CRUD API and admin overview island
- Add mobility overview island (Liste, Kanban, Detail CRUD views)
- Add contract PDF upload/download endpoints for mobilites
- Add light/dark theme toggle in header
- Add employeeOnly flag to hide entire modules from students (admin, students, stages)
- Add read-only GET endpoints for modules/ues/ue-modules in notes module
- Add [slug].tsx catch-all routes for direct URL navigation
- Replace old mobility table with mobilites + stages schema (migration 0004)
- Allow students to create mobilites and upload contracts
- Redirect authenticated users from / to /apps catalog

static/theme.js

@@ -0,0 +1,29 @@
+(function () {
+  var t = localStorage.getItem("theme");
+  if (t) document.documentElement.style.colorScheme = t;
+
+  document.addEventListener("click", function (e) {
+    var btn = e.target.closest("#theme-toggle");
+    if (!btn) return;
+    var cs = getComputedStyle(document.documentElement).colorScheme;
+    var isDark = cs === "dark" ||
+      (!cs || cs === "light dark") &&
+        matchMedia("(prefers-color-scheme:dark)").matches;
+    var next = isDark ? "light" : "dark";
+    document.documentElement.style.colorScheme = next;
+    localStorage.setItem("theme", next);
+    btn.querySelector("span").textContent = next === "dark"
+      ? "light_mode"
+      : "dark_mode";
+  });
+
+  document.addEventListener("DOMContentLoaded", function () {
+    var btn = document.getElementById("theme-toggle");
+    if (!btn) return;
+    var cs = getComputedStyle(document.documentElement).colorScheme;
+    var isDark = cs === "dark" ||
+      (!cs || cs === "light dark") &&
+        matchMedia("(prefers-color-scheme:dark)").matches;
+    btn.querySelector("span").textContent = isDark ? "light_mode" : "dark_mode";
+  });
+})();