Add dictation and streamed read-aloud to the chat

- A mic in the composer records, transcribes with the bot's
  transcription model (api/admin/bots/transcribe) and writes the text
  into the input at the cursor to edit before sending. Escape cancels;
  recordings aren't stored.
- Every bot message has a read-aloud button that plays
  GET api/admin/bots/speak-message, streamed from the provider as the
  audio is made, so it starts in a couple of seconds. One message plays
  at a time; only the user's own sessions can be read.
- Both buttons stay visible but disabled while the bot has no model for
  them (bot_voice page prop).
- Transcription and speech no longer pick free models automatically:
  they're not available until a model is picked, and Settings says so.
  Transcription lists speech-to-text models only, so the chat-model
  transcript path and its missing-audio check are gone.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/harness/audio/speak-text.ts

@@ -22,8 +22,10 @@ type Params = {
 };
 
 /**
- * Reads text aloud with the speech harness. Uses the voice picked in the
- * speech setting when the model has it, else the model's first voice.
+ * Reads text aloud with the speech harness, as a stream that can play
+ * while the rest is still being made. Falls back to the next model only
+ * until one answers. Uses the voice picked in the speech setting when the
+ * model has it, else the model's first voice.
  */
 export default async function speakText({
     text,