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.

simtrace2_pysniff/__init__.py

@@ -6,7 +6,7 @@ from .gsmtap import GsmtapSender, GsmtapReceiver
 from .pcap import PcapWriter
 from .dump import FileDumper, format_message
 
-__version__ = '1.1.0'
+__version__ = '1.2.0'
 __all__ = [
     'SniffSession',
     'DeviceDisconnected',