feat(library): support LIBRARY_BROWSE_ROOT to restrict file path access

Introduce a LIBRARY_BROWSE_ROOT environment variable (defaulting to the
current directory) to restrict directories users can browse and select when
setting up library paths. Add a PathPolicyService on the backend to enforce
path containment checks securely, and update the frontend FolderPickerModal
to prevent navigating above the configured browse root.

Closes #567

server/test/e2e/authorization-matrix/route-inventory/manifest.json

@@ -1,5 +1,5 @@
 {
-  "totalRoutes": 353,
+  "totalRoutes": 354,
   "byPermission": {
     "(none)": 161,
     "Permission.ManageMetadataConfig": 32,
@@ -7,7 +7,7 @@
     "Permission.LibraryEditMetadata": 31,
     "Permission.BookDockAccess": 17,
     "Permission.ManageAppSettings": 23,
-    "Permission.ManageLibraries": 13,
+    "Permission.ManageLibraries": 14,
     "Permission.ManageUsers": 10,
     "Permission.ManageEmail": 8,
     "Permission.KoboSync": 7,