feat: add collapse/expand controls for file tree and file diffs; compactible with new/old pr diff page

[rrbe]

Summary

• Add two toggle buttons to PR/commit/compare diff pages:
  • Collapse/Expand tree — inside the file tree sidebar, collapses or expands all folder nodes with a single click (handles nested directories by processing deepest-first)
  • Collapse/Expand all files — in the right toolbar area, collapses or expands all file diff sections at once
• Both buttons use GitHub Octicon icons (fold/unfold) for visual clarity
• Single settings toggle controls both buttons (feature-pr-collapse-expand, default ON)
• Works on PR Files Changed, commit detail, and compare pages (tree toggle is PR-only since only PR pages have the file tree sidebar)
• Compatible with both old and new GitHub "files changed" UI — uses TreeFolder/DiffFile interfaces to abstract DOM differences, falling back to old selectors (file-tree, .diffbar, .file-header) when the new feature preview is not enabled

Test plan

• On a PR Files Changed page (new experience), verify "Collapse tree / Expand tree" button appears in the file tree sidebar and toggles all folder nodes in one click
• On a PR Files Changed page (old experience, without feature preview), verify the same tree toggle works with ActionList-based file tree
• On a PR Files Changed page, verify "Collapse all files / Expand all files" button appears in the toolbar and toggles all file diffs
• On a commit page, verify "Collapse all files / Expand all files" button appears and works
• On a compare page, verify "Collapse all files / Expand all files" button appears and works
• In extension settings, verify toggling off "Collapse/Expand All Files" removes both buttons

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