Polish review feedback sidebar and inline threads

[selfcontained]

Summary

• polish the real review-feedback sidebar with clearer hierarchy, connected thread structure, token-based diff/message styling, animated replies, and state-specific actions
• add compact inline review threads to the Changes diff, including Markdown conversations, reply and resolution interactions, error feedback, keyboard shortcuts, and action-specific pending states
• navigate sidebar locations to the exact inline thread, consume focus once, and close mobile review/file-tree overlays so the diff remains usable
• extend review feedback resolution handling and seeded data with the Open, Fixed, and Dismissed state model

Validation

• pnpm run check
• pnpm run finalize:web
• Playwright validation on the real seeded agent/review route at desktop and mobile widths
• frame-by-frame reply transition validation
• author-requested frontend-ux-review persona: approved in round 2 with no remaining concerns

Full E2E was intentionally deferred during active visual iteration.