feat(file-manager): split "All Files" into internal-only + External views (#1621)

Reporter linked a NAS and the auto-imported files drowned their own
  Bambuddy uploads in the "All Files" sidebar listing. There was no filter
  to escape it — only per-folder clicks. Restore the pre-external semantics:
  "All Files" now lists managed-storage files only. The combined
  across-every-external view moves to a new sibling sidebar entry,
  "External", that only appears when at least one external folder is linked.

  Backend: GET /api/v1/library/files gains internal_only and external_only
  query flags. Filter is on LibraryFile.is_external. Both flags set is a
  400, not a silent pick-one.

  Frontend: new topLevelView state on FileManagerPage (default internal);
  the query passes the scope only when selectedFolderId is null. Mobile
  selector dropdown uses __top:internal / __top:external sentinels so the
  same state round-trips through option values. Empty-state copy
  distinguishes internal-empty from external-empty.

frontend/src/api/client.ts

@@ -5541,7 +5541,12 @@ export const api = {
   getLibraryFoldersByArchive: (archiveId: number) =>
     request<LibraryFolder[]>(`/library/folders/by-archive/${archiveId}`),
 
-  getLibraryFiles: (folderId?: number | null, includeRoot = true, projectId?: number) => {
+  getLibraryFiles: (
+    folderId?: number | null,
+    includeRoot = true,
+    projectId?: number,
+    scope?: 'internal' | 'external',
+  ) => {
     const params = new URLSearchParams();
     if (folderId !== undefined && folderId !== null) {
       params.set('folder_id', String(folderId));
@@ -5550,6 +5555,8 @@ export const api = {
       params.set('project_id', String(projectId));
     }
     params.set('include_root', String(includeRoot));
+    if (scope === 'internal') params.set('internal_only', 'true');
+    else if (scope === 'external') params.set('external_only', 'true');
     return request<LibraryFileListItem[]>(`/library/files?${params}`);
   },
   getLibraryFile: (id: number) => request<LibraryFile>(`/library/files/${id}`),