fix(tui): clear TTS env var on voice off, and add TTS indicator to status bar

Bug 1: /voice off in TUI mode did not clear HERMES_VOICE_TTS,
leaving TTS stuck ON with no way to disable it (the voice.toggle
tts handler requires voice mode to be ON).

Bug 2: TUI status bar only showed 'voice on/off' without any
indication of whether TTS speech output is active, because the
frontend never tracked voiceTts state.

- tui_gateway/server.py: clear HERMES_VOICE_TTS when voice is turned off
- ui-tui/src/app/useMainApp.ts: add voiceTts state, thread setVoiceTts
  through voice contexts, display [tts] in status bar
- ui-tui/src/app/slash/commands/session.ts: sync tts from voice.toggle response
- ui-tui/src/app/interfaces.ts: add setVoiceTts to all voice context interfaces

ui-tui/src/app/interfaces.ts

@@ -216,6 +216,7 @@ export interface InputHandlerContext {
     setProcessing: StateSetter<boolean>
     setRecording: StateSetter<boolean>
     setVoiceEnabled: StateSetter<boolean>
+    setVoiceTts: StateSetter<boolean>
   }
   wheelStep: number
 }
@@ -254,6 +255,7 @@ export interface GatewayEventHandlerContext {
     setProcessing: StateSetter<boolean>
     setRecording: StateSetter<boolean>
     setVoiceEnabled: StateSetter<boolean>
+    setVoiceTts: StateSetter<boolean>
   }
 }
 
@@ -296,6 +298,7 @@ export interface SlashHandlerContext {
   voice: {
     setVoiceEnabled: StateSetter<boolean>
     setVoiceRecordKey: (v: ParsedVoiceRecordKey) => void
+    setVoiceTts: StateSetter<boolean>
   }
 }