feat(webchat): add server-side dictation (#76021)

Summary:
- This PR adds WebChat server-side dictation through a new authenticated `chat.transcribeAudio` Gateway RPC, MediaRecorder composer controls, docs/changelog updates, and focused gateway/UI tests.
- Reproducibility: yes. Current main reproduces the missing feature by inspection: the Gateway method list, write scopes, docs, and WebChat voice-control test have no `chat.transcribeAudio` server-dictation path.

ClawSweeper fixups:
- Included follow-up commit: feat(webchat): add server-side dictation
- Included follow-up commit: fix(clawsweeper): address review for automerge-openclaw-openclaw-7602…

Validation:
- ClawSweeper review passed for head 850571380a.
- Required merge gates passed before the squash merge.

Prepared head SHA: 850571380a
Review: https://github.com/openclaw/openclaw/pull/76021#issuecomment-4363514226

Co-authored-by: Peter Steinberger <steipete@gmail.com>
Co-authored-by: clawsweeper <274271284+clawsweeper[bot]@users.noreply.github.com>

docs/nodes/audio.md

@@ -17,6 +17,7 @@ title: "Audio and voice notes"
   5. On success, it replaces `Body` with an `[Audio]` block and sets `{{Transcript}}`.
 - **Command parsing**: When transcription succeeds, `CommandBody`/`RawBody` are set to the transcript so slash commands still work.
 - **Verbose logging**: In `--verbose`, we log when transcription runs and when it replaces the body.
+- **Control UI dictation**: The Chat composer can send a browser-recorded microphone clip to `chat.transcribeAudio`. That Gateway RPC writes the clip to a temporary local file, runs this same audio transcription pipeline, returns draft text to the browser, and deletes the temporary file. It does not create an agent run by itself.
 
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