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/pages/admin/bots/[bot_id]/(data)/speak-message-context.ts

@@ -0,0 +1,13 @@
+import React from "react";
+import type { SpeakMessage } from "../(hooks)/use-speak-message";
+
+/**
+ * The conversation's read-aloud, shared by every bot message's play
+ * button.
+ */
+export const SpeakMessageContext = React.createContext<SpeakMessage>({
+    enabled: false,
+    message_id: null,
+    state: "idle",
+    toggle: () => {},
+});