Add --capture disabled mode + capture-mode status in PWA

Server:
- --capture accepts 'disabled'; no backend is created, capture_mode exposed
- /api/status returns capture_mode; capture start/stop return 403 when
  disabled; latest/status/delete_session are None-safe

PWA:
- captureMode state; conn-status shows server version
- new capture-mode line (Settings) explaining gsmtap/direct/disabled
- hide capture + New capture UI when disabled; trace empty-state shows
  'Capture unavailable'
- 7 new EN/RU i18n keys

Version 1.2.0 (server + PWA); tests: 111.

frontend/manifest.json

@@ -2,7 +2,7 @@
   "name": "SIMtrace Analyser",
   "short_name": "SIMtrace",
   "description": "PWA frontend for browsing SIMtrace2 APDU traces",
-  "version": "1.1.0",
+  "version": "1.2.0",
   "start_url": "index.html",
   "display": "standalone",
   "background_color": "#0f172a",