DMS redesign: 3-column explorer layout with toolbar, source tree, file details panel

- Add backend endpoints: GET /dms/files, GET /dms/folders/{id}/files
- New SourceTree component: sources (Meine Dateien, Geteilte Ordner, Externe Speicher)
- New FileExplorer component: 5 view modes (list, table, icons-sm/md/lg)
- New FileDetails component: right panel with file metadata and actions
- Rewrite Dms.tsx: PluginToolbar + 3-column ResizablePanel layout
- Update DmsFile type: size_bytes + uploaded_by (backward compat aliases)
- Add 60+ i18n keys for de/en

frontend/src/api/dms.ts

@@ -25,15 +25,20 @@ export interface DmsFile {
   folder_id: string | null;
   name: string;
   mime_type: string;
-  size: number;
+  size_bytes?: number;
   storage_path: string;
   checksum?: string | null;
   deleted_at: string | null;
-  created_by: string;
+  uploaded_by?: string;
   created_at?: string | null;
   updated_at?: string | null;
   shared_with?: string[];
   permissions?: FilePermission[];
+  access_level?: string;
+  /** @deprecated Use size_bytes instead */
+  size?: number;
+  /** @deprecated Use uploaded_by instead */
+  created_by?: string;
 }
 
 export interface FilePermission {
@@ -189,6 +194,18 @@ export function getSharedWithMe(): Promise<DmsFile[]> {
   return apiGet<DmsFile[]>('/dms/shared-with-me');
 }
 
+export function fetchSharedWithMe(): Promise<DmsFile[]> {
+  return getSharedWithMe();
+}
+
+export function fetchFilesByFolder(folderId: string): Promise<DmsFile[]> {
+  return apiGet<DmsFile[]>(`/dms/folders/${folderId}/files`);
+}
+
+export function fetchAllFiles(): Promise<DmsFile[]> {
+  return apiGet<DmsFile[]>('/dms/files');
+}
+
 // ─── Bulk Operations ───────────────────────────────────────────────────────
 
 export function bulkMoveFiles(payload: BulkMovePayload): Promise<void> {