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/catalog/model-serves-harness.ts

@@ -13,7 +13,8 @@ type Params = {
  * Whether a model can do a harness's job:
  * - text: reads and writes text, and calls tools (bots work through tools)
  * - vision: reads images, writes text
- * - transcription: a speech-to-text model, or a chat model that hears audio
+ * - transcription: a speech-to-text model (never a chat model that hears
+ *   audio: free ones often drop it)
  * - speech: a text-to-speech model
  * - image: makes images
  */
@@ -31,10 +32,7 @@ export default function modelServesHarness({ model, harness }: Params) {
             return input.has("image") && output.has("text");
 
         case "transcription":
-            return (
-                output.has("transcription") ||
-                (input.has("audio") && output.has("text"))
-            );
+            return output.has("transcription");
 
         case "speech":
             return output.has("speech");