feat: virtualize ReviewSidebar file list for >50 items with scroll preservation

Adds windowed rendering to FilesView in ReviewSidebar.tsx using
react-window 2.x (List component). File lists with more than 50 rows
render only visible items, keeping the DOM lean for large diffs.

- Install react-window 2.x and @types/react-window in apps/web
- Flatten directory-grouped file tree into a typed Row[] array via useMemo
- Use VariableSizeList-equivalent react-window 2.x List with rowHeight fn
  (32px for dir-headers, 40px for file rows); falls back to plain DOM
  render for ≤50 rows to avoid overhead on small diffs
- Directories are collapsible: clicking the dir-header toggles collapse,
  removing its file rows from the Row[] and from the virtual list
- Preserve sidebar scroll offset across Files ↔ Commits tab switches via
  filesScrollOffsetRef passed from ReviewSidebar into FilesView
- Clicking a file calls listRef.scrollToRow({ index, align: "smart" })
  to keep the clicked row visible in the virtual list
- Root-level files (directory === "") render without a dir-header,
  preserving existing behavior
- Add resolve.dedupe for react/react-dom in vitest.config.ts to prevent
  duplicate-React errors after local workspace package installation
- Add 6 Vitest + RTL tests covering: large-list DOM count, small-list
  fallback, collapse, re-expand, tab-switch smoke, root-level files

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/frontend.md

@@ -14,6 +14,7 @@
 | Tiptap | Rich text editor (ProseMirror-based) |
 | Lucide | Icon library |
 | Geist Sans/Mono | Typography (variable fonts in `public/fonts/`) |
+| react-window 2.x | Virtualized list rendering for large file trees in ReviewSidebar |
 
 ## Design System (v2)
 
@@ -115,7 +116,7 @@ The initiative detail page has three tabs managed via local state (not URL param
 |-----------|---------|
 | `ReviewTab` | Review tab container — orchestrates header, diff, sidebar, and preview. Phase-level review has threaded inline comments (with reply support) + Request Changes; initiative-level review has Request Changes (summary prompt) + Push Branch / Merge & Push |
 | `ReviewHeader` | Consolidated toolbar: phase selector pills, branch info, stats, preview controls, approve/reject actions |
-| `ReviewSidebar` | VSCode-style icon strip (Files/Commits views) with file list, root-only comment counts, and commit navigation |
+| `ReviewSidebar` | VSCode-style icon strip (Files/Commits views) with file list, root-only comment counts, and commit navigation. FilesView uses react-window 2.x `List` for virtualized rendering when the row count exceeds 50 (dir-headers + file rows). Scroll position is preserved across Files ↔ Commits tab switches. Directories are collapsible. Clicking a file scrolls the virtual list to that row. |
 | `DiffViewer` | Unified diff renderer with threaded inline comments (root + reply threads) |
 | `CommentThread` | Renders root comment with resolve/reopen + nested reply threads (agent replies styled with primary border). Inline reply form |
 | `ConflictResolutionPanel` | Merge conflict detection + agent resolution in initiative review. Shows conflict files, spawns conflict agent, inline questions, re-check on completion |