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

databases/migrations/meta/_journal.json

@@ -29,6 +29,13 @@
       "when": 1777155028711,
       "tag": "0003_add_session2_and_malus",
       "breakpoints": true
+    },
+    {
+      "idx": 4,
+      "version": "7",
+      "when": 1777155028712,
+      "tag": "0004_add_stages_and_mobilites",
+      "breakpoints": true
     }
   ]
 }