fix(tts): add missing OpenAI TTS voices (ballad, cedar, marin, verse)

Fixes #2393

Add the following OpenAI TTS voices to the OPENAI_TTS_VOICES allowlist:
- ballad
- cedar
- marin
- verse

These voices are listed in OpenAI's /audio/speech API reference but were
missing from the main TTS module. The extensions/voice-call module already
had these voices; this PR syncs src/tts/tts.ts to match.

Note: juniper mentioned in the issue is a ChatGPT app-only voice and is
not available via the TTS API, so it is intentionally not included.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/tts/tts.ts

@@ -765,13 +765,17 @@ const isCustomOpenAIEndpoint = OPENAI_TTS_BASE_URL !== "https://api.openai.com/v
 export const OPENAI_TTS_VOICES = [
   "alloy",
   "ash",
+  "ballad",
+  "cedar",
   "coral",
   "echo",
   "fable",
-  "onyx",
+  "marin",
   "nova",
+  "onyx",
   "sage",
   "shimmer",
+  "verse",
 ] as const;
 
 type OpenAiTtsVoice = (typeof OPENAI_TTS_VOICES)[number];