feat: CommentPopover for better comment writing UX

[backnotprop]

Summary

• Replaces the cramped toolbar textarea (max-w-80 max-h-32) and awkward global comment inline form with a new CommentPopover component
• Popover mode (w-96) provides a wider, auto-growing textarea with proper padding
• Expand button opens a full dialog mode (max-w-xl) for long-form writing
• Simplified AnnotationToolbar to menu-only — removed the entire input step (~100 lines)
• Cmd/Ctrl+Enter to submit, Enter for newline (GitHub/Linear comment pattern)
• Popover tracks anchor element position on scroll (stays pinned to highlighted content)
• Image attachments work in both popover and dialog modes

Test plan

• Select text → Comment → popover appears with proper size
• Type long comment → expand to dialog → text persists → submit creates annotation
• Type-to-comment (press key in menu step) → popover opens with char pre-filled
• Global comment button → popover opens → submit creates global annotation
• Code block hover → Comment → popover opens → submit creates annotation
• Scroll while popover is open → popover stays pinned to content
• Escape in dialog → collapse to popover. Escape in popover → close
• Click outside popover → closes

Closes #219

🤖 Generated with Claude Code

[backnotprop]

Code review

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

🤖 Generated with Claude Code

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