v2.5.3 — Admin update checker & one-click self-update

- Add version check against GitHub releases in admin dashboard
- Show amber banner when a newer version is available
- One-click update: git pull + npm install + auto-restart
- Confirmation dialog with backup recommendation and data safety info
- Dark mode support for update banner
- Fix fresh DB migration: initial schema now includes all columns
- i18n: English + German translations for all update UI

client/src/api/client.js

@@ -129,6 +129,8 @@ export const adminApi = {
   updateOidc: (data) => apiClient.put('/admin/oidc', data).then(r => r.data),
   addons: () => apiClient.get('/admin/addons').then(r => r.data),
   updateAddon: (id, data) => apiClient.put(`/admin/addons/${id}`, data).then(r => r.data),
+  checkVersion: () => apiClient.get('/admin/version-check').then(r => r.data),
+  installUpdate: () => apiClient.post('/admin/update', {}, { timeout: 300000 }).then(r => r.data),
 }
 
 export const addonsApi = {