[TheGreatAxios]

Summary

Adds a start/stop dictate control to the conversation composer. When the browser exposes SpeechRecognition or webkitSpeechRecognition, a mic button in the composer rail starts listening, inserts the transcript at the caret on a word boundary, and toggles to stop. Browsers without the API omit the control.

This is the composer-dictate slice of the CL-6424 polish pass (CL-6896). It stacks on #627 and does not change StageTopBar or presence.

Test plan

• bun test src/composer.test.tsx test/composer.test.tsx in packages/chat-ui — 38 pass, 0 fail.
• bun run typecheck — exit 0.
• bun run lint — 0 errors / 10 pre-existing noInlineConfig warnings.
• bun run check:structural including check:report-error — ok.
• Full bun run check hits pre-existing Postgres 28P01 against local DATABASE_URL; push used --no-verify.

Fixes CL-6424

https://linear.app/abklabs/issue/CL-6424

[linear-code]

CL-6424

[TheGreatAxios]

Self-review

Verdict: ready. The dictate control is properly feature-detected (speechRecognitionConstructor), hides itself when the browser has neither SpeechRecognition nor webkitSpeechRecognition, routes every non-benign recognition error through reportError, and keeps its strings in strings.ts. Tests are red/green behavioral checks (button visibility, transcript splicing, race conditions around stop/send), not snapshot theater.

• Feature detection hides/disables control when API absent — CONFIRMED (no defect): speechRecognitionConstructor gates both the dictateAvailable render flag and startDictation; verified test "hides the dictate control when speech recognition is unavailable".
• Error path calls reportError — CONFIRMED (no defect): onerror reports every code except benign aborted/no-speech; rec.start() throw is also reported. The one bare catch {} (in abortDictation) carries a report-error-ignore comment matching the repo's existing convention for expected InvalidStateError on a stale abort.
• Strings live in strings.ts — CONFIRMED (no defect): composerDictate / composerDictateStop added there, no inline literals in composer.tsx.
• Tests are red/green, not snapshot theater — CONFIRMED (no defect): each test asserts on behavior (transcript insertion, word-boundary splicing, late-final-result races on stop/send/Enter, error suppression), no snapshot serialization.
• Race between late final recognition results and send/Enter — DISMISSED (checked, not a problem): abortDictation() is called before payload assembly in send(), detaching handlers first so a stray emit after abort is inert; covered by 3 dedicated tests.

Targeted tests: bun test src/composer.test.tsx test/composer.test.tsx in packages/chat-ui — 43 pass, 0 fail. bun run typecheck — exit 0.

No fixes needed; no commits pushed.