feat(ui): draggable comment popover and annotation toolbar

[backnotprop]

Summary

• Adds a shared useDraggable hook that makes fixed-position elements movable by their header
• Integrates into AnnotationToolbar (code review line comments) and CommentPopover (plan review + file-level code review comments)
• Resolves user reports of awkward popover positioning near viewport edges/bottom

How it works

• Drag the header/title bar of any comment popover to reposition it
• 3px movement threshold prevents interfering with button clicks in the header
• Once dragged, scroll/resize auto-repositioning pauses (user is in control)
• Position resets when a new annotation opens

Test plan

• bun run dev:review — select diff lines, drag toolbar header, verify repositioning
• Verify header buttons (cancel, expand, close) still work without triggering drag
• Verify Esc / Cmd+Enter keyboard shortcuts still work
• Select new lines → verify toolbar resets to auto position
• Test near viewport edges (top, bottom, left, right)
• Plan review: select text → drag comment popover header
• Verify scroll doesn't fight dragged position
• Expand to dialog and back → verify drag resets

[backnotprop]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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