feat(version): warn when the server was updated but not restarted (#898)

When the package is updated on disk but the long-lived server process is
not restarted, the new frontend bundle (served from disk) talks to the
old running backend. New DB-backed features then fail silently — e.g.
deleting/archiving a session appears to do nothing — because the new
schema/routes only take effect on restart.

Nothing currently detects this skew: useVersionCheck only compares the
frontend's build-time version against the latest GitHub release.

This exposes the running server's version (captured once at startup) via
/health, compares it to the frontend's build-time version in
useVersionCheck, and shows a "restart required" banner in the sidebar
(and a small indicator in the collapsed sidebar) when they differ.

- server: add `version` (RUNNING_VERSION, read once at startup) to /health
- useVersionCheck: return `restartRequired` / `runningVersion`
- SidebarFooter / SidebarCollapsed: surface a restart-required banner
- i18n: add `version.restartRequired` to all 10 sidebar locales

Verified with `tsc --noEmit` (client + server) and eslint.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Simos Mikelatos <simosmik@gmail.com>

src/i18n/locales/de/sidebar.json

@@ -115,7 +115,8 @@
     "restoreSessionError": "Fehler beim Wiederherstellen der Sitzung. Bitte erneut versuchen."
   },
   "version": {
-    "updateAvailable": "Update verfügbar"
+    "updateAvailable": "Update verfügbar",
+    "restartRequired": "Update installiert – zum Anwenden Server neu starten"
   },
   "search": {
     "modeProjects": "Projekte",