feat(ui): email row polish + custom checkbox
- Add onOpen prop to EmailRow (calls parent handler when provided,
falls back to opening in Gmail); keep explicit Open-in-Gmail action.
- New accessible Checkbox primitive (Radix + design tokens, focus ring),
exported from ui barrel; replaces bare input in the select cell.
- Row polish: consistent height, vertical rhythm, single hover state,
clearer hierarchy (prominent subject, muted sender/snippet), one-line
ellipsis snippet.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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