Add shared file storage with folder tree, permissions, and file manager UI

New document hub for centralized file storage — replaces scattered
drives and USB sticks for non-technical SMBs. Three new tables:
storage_folder (nested hierarchy), storage_folder_permission (role
and user-level access control), storage_file.

Backend: folder CRUD with nested paths, file upload/download via
signed URLs, permission checks (view/edit/admin with inheritance
from parent folders), public/private toggle, breadcrumb navigation,
file search.

Frontend: two-panel file manager — collapsible folder tree on left,
icon grid view on right. Folder icons by type, file size display,
upload button, context menu for download/delete. Breadcrumb nav.
Files sidebar link added.

packages/backend/src/db/migrations/meta/_journal.json

@@ -155,6 +155,13 @@
       "when": 1774810000000,
       "tag": "0021_remove_company_scoping",
       "breakpoints": true
+    },
+    {
+      "idx": 22,
+      "version": "7",
+      "when": 1774820000000,
+      "tag": "0022_shared_file_storage",
+      "breakpoints": true
     }
   ]
 }