Add committed-only option to Changes diff

[selfcontained]

Summary

• add a default-on, persisted setting to include uncommitted changes in the Changes diff and stats
• keep diff content, file views, stats, and SSE updates synchronized across the selected mode
• stabilize the Changes header badge layout and add a concise inline tip on diff settings
• widen the settings popover so labels stay on one line

Validation

• pnpm run check
• pnpm run finalize:web
• focused tip registry tests (11/11)
• browser verified both modes, persistence, synchronized requests/stats, stable header layout, and inline tip
• full E2E suite passed before the final copy-only adjustment (168 passed, 12 skipped); not rerun for copy per request