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/pages/DmsTrash.tsx

@@ -59,11 +59,12 @@ export function DmsTrashPage() {
       key: 'size',
       header: t('dms.fileSize'),
       sortable: true,
-      accessor: (file) => file.size,
+      accessor: (file) => file.size_bytes ?? file.size ?? 0,
       render: (file) => {
-        if (file.size < 1024) return `${file.size} B`;
-        if (file.size < 1024 * 1024) return `${(file.size / 1024).toFixed(1)} KB`;
-        return `${(file.size / (1024 * 1024)).toFixed(1)} MB`;
+        const sz = file.size_bytes ?? file.size ?? 0;
+        if (sz < 1024) return `${sz} B`;
+        if (sz < 1024 * 1024) return `${(sz / 1024).toFixed(1)} KB`;
+        return `${(sz / (1024 * 1024)).toFixed(1)} MB`;
       },
     },
     {