feat: PR review comments in Changes sidebar and diff view

[eugenioenko]

Summary

• Add GitHub API functions to fetch and post PR review comments (both inline and general)
• Integrate a Conversation section into the Changes sidebar for PR groups, showing general comments with an input to add new ones
• Show comment count badges next to PR files that have inline review comments
• Render inline comments as styled blocks within the diff view, placed between diff lines at their corresponding positions
• Add two new theme-aware styles (StyleCommentBg, StyleCommentAuthor) following the existing theme pipeline
• Wire async comment fetching into the existing PR load flow and auto-refresh after posting

Design approach

No new panels, overlays, or widgets. Comments are integrated directly into the existing Changes widget (sidebar) and DiffViewWidget (editor area), matching existing patterns:

• New changesItemKind values (itemConvoSection, itemComment, itemCommentInput) extend the virtual item list
• diffDisplayRow interleaves comment blocks with diff lines in the display row list
• Callbacks (OnAddComment, OnViewComment) follow the existing callback-driven architecture
• Async results (PrCommentAddResult, PrCommentsRefreshResult) use the standard PostEvent(EventInterrupt) pattern

Test plan

• Unit tests for FormatCommentTime, CommentsForFile, GeneralComments, FileCommentCounts
• All existing tests pass (go test ./... -- the only failure is pre-existing TestLoadNoFiles)
• Build succeeds (make build)
• Manual: open a PR with gh pr open <url>, verify conversation section appears
• Manual: verify comment count badges show next to files with inline comments
• Manual: open a PR diff, verify inline comments render between diff lines
• Manual: add a general comment via the sidebar input, verify it posts and refreshes

🤖 Generated with Claude Code