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

routes/(_components)/Header.tsx

@@ -11,6 +11,14 @@ export default function Header(props: HeaderProps) {
       <nav>
         <a href="/apps" f-client-nav={false}>Catalog</a>
         <a href={`/log${props.link}`} f-client-nav={false}>Log {props.link}</a>
+        <button
+          id="theme-toggle"
+          type="button"
+          title="Changer de theme"
+          style="background:none;border:none;cursor:pointer;font-size:1.2rem;padding:0;line-height:1;"
+        >
+          <span class="material-symbols-outlined">dark_mode</span>
+        </button>
       </nav>
     </header>
   );