feat: add PR review comments sidebar panel

[eugenioenko]

Summary

• Add a new Reviews sidebar panel that displays PR review comments when a pull request is loaded
• Fetch both inline review comments and general issue comments from GitHub via gh api
• Display comments grouped by file (inline) with a "General" section for issue-level comments
• Add input widget at the bottom for posting new general comments to the PR
• Wire into existing PR loading flow: comments are fetched automatically when a PR is opened via Changes panel

Changes

• internal/github/github.go -- New PRComment type, FetchPRComments, AddPRComment, AddPRInlineComment functions
• internal/ui/reviews_widget.go -- New ReviewsWidget using BaseWidget + SelectableList pattern with scrollbar, input, and file grouping
• internal/app/widgets.go -- Register Reviews panel in sidebar via sidebar.AddPanel("reviews", "Reviews", reviews)
• internal/app/app.go -- Add Reviews field to App struct
• internal/app/callbacks.go -- Wire OnOpenFile (navigate to file:line) and OnAddComment (post comment via GitHub API)
• internal/app/pr.go -- New PRCommentsFetchResult and PRCommentAddResult event types, FetchPRComments/RefreshPRComments methods
• internal/app/eventloop.go -- Handle comment fetch/add results; auto-fetch comments when PR is loaded
• internal/app/commands_git.go -- Register reviews.refresh command
• tests/e2e/sidebar_test.go -- Update overflow test for 4-panel sidebar

Test plan

• Unit tests for PRComment type and JSON parsing (internal/github/github_test.go)
• Unit tests for ReviewsWidget creation, comment grouping, rendering, selection styles, callbacks, and loading state (internal/ui/reviews_widget_test.go)
• E2E sidebar overflow test updated for 4 panels (tests/e2e/sidebar_test.go)
• make build succeeds
• make test passes (pre-existing config test failure unrelated)
• Manual: open a PR URL, verify Reviews tab appears and shows comments
• Manual: click an inline comment to navigate to the file/line
• Manual: type a comment in the input and press Enter to post it

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