feat: voice and model pickers with sync-on-join (Phase 4)

- Voices.swift: 13 Azure voices, default en-US-Ava
- SettingsStore: persist voiceID/modelID, fallback to default voice
- VoiceSession: sync set_voice/set_model on agent_ready, selectVoice/selectModel, track models + model_changed
- SettingsView: voice and model picker sheets
- ControlChannel: publish via publish(data:) reliable data channel instead of sendText DataStream (agent listens on legacy data_received; DataStream text was dropped)

Sources/App/SettingsStore.swift

@@ -29,7 +29,8 @@ final class SettingsStore: ObservableObject {
     init() {
         let d = UserDefaults.standard
         serverURL = d.string(forKey: Keys.serverURL) ?? "https://192.168.86.2:9178"
-        voiceID = d.string(forKey: Keys.voiceID) ?? ""
+        let storedVoice = d.string(forKey: Keys.voiceID)
+        voiceID = Voices.all.contains { $0.id == storedVoice } ? (storedVoice ?? "") : Voices.defaultID
         modelID = d.string(forKey: Keys.modelID)
         textMode = d.bool(forKey: Keys.textMode)
         volume = d.object(forKey: Keys.volume) as? Double ?? 0.8