Add cancel control for in-progress dictation transcription

[Samigos]

Summary

• add an explicit cancel action while dictation is in the processing/transcribing state
• wire cancel dictation through app layout/composer/workspace surfaces
• update composer mic control to show and trigger "Cancel transcription" during processing
• add focused test coverage for processing cancel behavior

Validation

• npm run typecheck
• npm run test -- src/features/composer/components/ComposerInput.dictation.test.tsx src/features/composer/components/ComposerSend.test.tsx src/features/composer/components/ComposerInput.attachments.test.tsx
• npm run test

[Samigos]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

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