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>

pnpm-lock.yaml

@@ -220,6 +220,9 @@ importers:
       react-image-crop:
         specifier: ^11.0.10
         version: 11.0.10(react@19.2.6)
+      react-resizable-panels:
+        specifier: ^3.0.6
+        version: 3.0.6(react-dom@19.2.6(react@19.2.6))(react@19.2.6)
       recharts:
         specifier: ^3.8.1
         version: 3.8.1(@types/react@19.2.14)(react-dom@19.2.6(react@19.2.6))(react-is@18.3.1)(react@19.2.6)(redux@5.0.1)
@@ -5139,6 +5142,12 @@ packages:
       '@types/react':
         optional: true
 
+  react-resizable-panels@3.0.6:
+    resolution: {integrity: sha512-b3qKHQ3MLqOgSS+FRYKapNkJZf5EQzuf6+RLiq1/IlTHw99YrZ2NJZLk4hQIzTnnIkRg2LUqyVinu6YWWpUYew==}
+    peerDependencies:
+      react: ^16.14.0 || ^17.0.0 || ^18.0.0 || ^19.0.0 || ^19.0.0-rc
+      react-dom: ^16.14.0 || ^17.0.0 || ^18.0.0 || ^19.0.0 || ^19.0.0-rc
+
   react-simple-animate@3.5.3:
     resolution: {integrity: sha512-Ob+SmB5J1tXDEZyOe2Hf950K4M8VaWBBmQ3cS2BUnTORqHjhK0iKG8fB+bo47ZL15t8d3g/Y0roiqH05UBjG7A==}
     peerDependencies:
@@ -10859,6 +10868,11 @@ snapshots:
     optionalDependencies:
       '@types/react': 19.2.14
 
+  react-resizable-panels@3.0.6(react-dom@19.2.6(react@19.2.6))(react@19.2.6):
+    dependencies:
+      react: 19.2.6
+      react-dom: 19.2.6(react@19.2.6)
+
   react-simple-animate@3.5.3(react-dom@19.2.6(react@19.2.6)):
     dependencies:
       react-dom: 19.2.6(react@19.2.6)