feat(ui): dictate into a composer that already has text, and in the dashboard

[iamnbutler]

Part of #38. Follows #473.

• Dashboard: the new-channel composer enables dictation (mic instead of mic={false}).
• Composer with text: the submit button stays the mic only while there's nothing to send. Once there's text or an attachment, or an agent is running, a mic button appears in the composer's options toolbar, next to the existing "Show tools" button. It uses the same Toolbar.Button/Button/IconMicMicro/Tooltip pattern as that button. While you dictate, the existing waveform takes that slot and the submit button becomes Stop, as before.
• Insert logic: dictation goes in at the cursor and replaces any selection, like macOS dictation. It adds a space before the dictated words when the text before them isn't whitespace, and a space after them when a word follows. Only the range captured when recording started is rewritten, so text around it is untouched.

Product calls: no new component and no shortcut. When the composer is empty the submit button is still the only mic, so there are never two mic buttons.

Smoke-tested with bun ace serve --loopback, an isolated ACE_HOME/ACE_CONFIG_HOME, and the built app in headless Playwright WebKit. The Mac screen was locked, so I couldn't drive the desktop WKWebView.

• In WebKit, webkitSpeechRecognition exists, but headless it never starts or errors. Clicking the dashboard's mic does switch the composer into recording (Stop button).
• A scripted stand-in recognizer, used only by the smoke script, fed transcripts into the dashboard composer:
  • "Fix the" + dictation gives "Fix the login bug", with interim results updating in place.
  • A late result after Stop is ignored.
  • Dictating with the cursor after "Fix" gives "Fix quickly the login bug".
  • Selecting "bug" and dictating gives "... login issue".
  • The empty composer still uses the submit mic.

Still open on #38: end-to-end check in the desktop app (first-run mic/speech prompts, glow during real recognition).

Built in Ace

Built with Claude Code outside Ace, in a session already running there; no Ace gap involved.