feat: hierarchical folder tree in code review sidebar

[backnotprop]

Summary

• Groups changed files by directory in the code review sidebar with collapsible folder nodes
• Folders show aggregate +N/-M stats and auto-expand when navigating with j/k keys
• Single-child directories collapse (e.g., src/utils/ as one node); single-directory diffs fall back to the existing flat list

Closes #161

Implementation

• buildFileTree.ts: Pure utility — trie construction, single-child collapsing, folder sorting (folders first, alphabetical)
• FileTreeNode.tsx: Recursive component following VaultBrowser's TreeNode pattern — depth-based padding, chevron expand/collapse, Tailwind-only (no new CSS classes)
• FileTree.tsx: Wires tree building via useMemo, manages expand/collapse state, pre-computes annotation counts as Map<string, number> for O(1) lookups

All existing functionality preserved: keyboard nav, viewed tracking, hide-viewed filter, diff dropdown, stats footer.

Test plan

• plannotator review from a repo with multi-directory changes — verify folder grouping, expand/collapse, aggregate stats
• plannotator review from a repo with single-directory changes — verify flat fallback (no folder wrapper)
• j/k keyboard nav — verify auto-expand of collapsed ancestor folders
• Hide viewed files — verify empty folders are hidden
• Diff type switch — verify tree rebuilds with all folders expanded

🤖 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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