Mobile dictation through the user's own environment server

[pedrozimmermannsoares]

Phone dictation is where T3 Code has no alternative: a third-party dictation app can't type into the mobile composer. Today the mic depends on Apple's on-device model, so Android has no dictation (#13603), iPhones before iOS 26 have none, and non-English speech is transcribed with the English model (#12743, pt-BR confirmed in the comments).

Proposal: let the mobile composer send the finished recording to the environment server it is already connected to, and transcribe it there with a local model. No API keys, no cloud service, and it uses hardware the user already runs. That keeps to the direction in #15422 and doesn't revisit the web/desktop scope decision in #11278.

I'd ship it in three separately approved PRs:

1. Server transcription with one opt-in local model (Parakeet), plus a mobile setting "Transcribe on: this phone / computer". Nothing changes by default.
2. Optional transcript cleanup with an editable prompt, through the text generation provider already configured.
3. Choosing the transcription model.

#14882 (@UtkarshUsername) already implements much of the server side for web/desktop. I'd reuse its approach for slice 1, with the author's agreement.

Is slice 1 in scope? If yes, I'll measure upload, transcription and cleanup latency on a real phone and post the numbers before opening the PR.