feat: add file-scoped comments to /plannotator-review

[sercantor]

Summary

• Add a file-scoped comment flow to /plannotator-review via a File Comment composer in each file header.
• Keep line-scoped comments unchanged and include file-scoped comments in Review Panel and exported feedback (### File Comment).
• Small cleanup: extract Review Panel sort helpers for readability while preserving behavior.

Closes #302

Test plan

• bun run build:review
• Manual end-to-end smoke test through /plannotator-review (feedback payload included ### File Comment).

[backnotprop]

Ah, nice work. I was just giving this a shot! Alright, I'll get it in review.

[sercantor]

feel free to dismiss if you have some other implementation in mind

[backnotprop]

No, all good. We can make this work.

[backnotprop]

Nice work on this — the scope field approach and ReviewPanel badge are great.

I pushed a small refactor (cf77b73) to swap the custom popover in FileHeader for the existing CommentPopover component from packages/ui/components/CommentPopover.tsx. This is the same popover used for global comments in the plan editor.

What changed:

• FileHeader now just has a button that passes its DOM element up via onFileComment(anchorEl)
• DiffViewer renders <CommentPopover> anchored to that element
• ~62 net lines removed

What you get for free by reusing it:

• Click-outside dismiss
• Escape key handling
• Expand-to-dialog mode (for longer comments)
• Image attachment support
• Scroll-aware anchor positioning
• Keyboard shortcut hints

Everything else from your PR (scope field, export format, ReviewPanel sorting) is kept as-is.