feat(deps): react-resizable-panels for docs hub desktop split

Docs hub's desktop sidebar is now drag-resizable. Mobile path is
unchanged — still uses the FolderTreeSidebar Sheet drawer.

- Extracted `FolderTreeBody` from `folder-tree-sidebar.tsx` so the
  same tree renders inside the mobile Sheet AND the desktop panel
  without forking the component.
- `FolderTreeSidebar` is now mobile-only (just the Sheet trigger);
  documents-hub composes the desktop layout itself.
- `<ResizablePanelGroup autoSaveId="documents-hub-split">` persists
  the user's chosen split width via localStorage automatically.
  Min 14% / max 40% defends against starvation.
- shadcn-style `<Resizable*>` primitives in `src/components/ui/`
  match the rest of the UI kit; uses react-resizable-panels v3
  (the v4 release renamed exports to `Group`/`Separator` and broke
  the shadcn convention — pinned v3 for now).

Verified: tsc clean, vitest 1315/1315, next build green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -97,6 +97,7 @@
     "react-email": "^6.1.3",
     "react-hook-form": "^7.75.0",
     "react-image-crop": "^11.0.10",
+    "react-resizable-panels": "^3.0.6",
     "recharts": "^3.8.1",
     "sharp": "^0.34.5",
     "socket.io": "^4.8.3",