✨(frontend) add subtitle control with transcription display

Kickstart frontend with first draft of subtitle control visible only
to users with appropriate feature flag enabled.

Opens new container at bottom of screen displaying transcription
segments organized by participant. Transcription segment handling was
heavily LLM-generated and will likely need refactoring and review to
simplify and enhance the implementation.

Initial implementation to begin testing subtitle functionality with
real transcription data from LiveKit agents.

src/frontend/src/locales/nl/rooms.json

@@ -168,6 +168,10 @@
       "raise": "Hand opsteken",
       "lower": "Hand laten zakken"
     },
+    "subtitles": {
+      "closed": "Ondertitels weergeven",
+      "open": "Ondertitels verbergen"
+    },
     "screenShare": {
       "start": "Scherm delen",
       "stop": "Stop schermdelen"