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>

package-lock.json

@@ -73,12 +73,14 @@
         "@tiptap/suggestion": "^3.19.0",
         "@trpc/client": "^11.9.0",
         "@trpc/react-query": "^11.9.0",
+        "@types/react-window": "^1.8.8",
         "class-variance-authority": "^0.7.1",
         "clsx": "^2.1.1",
         "geist": "^1.7.0",
         "lucide-react": "^0.563.0",
         "react": "^19.0.0",
         "react-dom": "^19.0.0",
+        "react-window": "^2.2.7",
         "sonner": "^2.0.7",
         "tailwind-merge": "^3.4.0",
         "tippy.js": "^6.3.7"
@@ -5198,6 +5200,15 @@
         "@types/react": "^19.2.0"
       }
     },
+    "node_modules/@types/react-window": {
+      "version": "1.8.8",
+      "resolved": "https://registry.npmjs.org/@types/react-window/-/react-window-1.8.8.tgz",
+      "integrity": "sha512-8Ls660bHR1AUA2kuRvVG9D/4XpRC6wjAaPT9dil7Ckc76eP9TKWZwwmgfq8Q1LANX3QNDnoU4Zp48A3w+zK69Q==",
+      "license": "MIT",
+      "dependencies": {
+        "@types/react": "*"
+      }
+    },
     "node_modules/@types/unist": {
       "version": "3.0.3",
       "resolved": "https://registry.npmjs.org/@types/unist/-/unist-3.0.3.tgz",
@@ -9131,6 +9142,16 @@
         }
       }
     },
+    "node_modules/react-window": {
+      "version": "2.2.7",
+      "resolved": "https://registry.npmjs.org/react-window/-/react-window-2.2.7.tgz",
+      "integrity": "sha512-SH5nvfUQwGHYyriDUAOt7wfPsfG9Qxd6OdzQxl5oQ4dsSsUicqQvjV7dR+NqZ4coY0fUn3w1jnC5PwzIUWEg5w==",
+      "license": "MIT",
+      "peerDependencies": {
+        "react": "^18.0.0 || ^19.0.0",
+        "react-dom": "^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/read-cache": {
       "version": "1.0.0",
       "resolved": "https://registry.npmjs.org/read-cache/-/read-cache-1.0.0.tgz",