feat(C): Phase C — Core UI prüfen, vervollständigen, testen

C-ERR-BOUNDARY: ErrorBoundary erweitert (trace_id, Fallback-UI, PluginErrorBoundary)
C-NOTIF: NotificationDropdown mit System-Channel-Link
C-DOCS: ApiDocs.tsx Seite (Swagger UI iframe)
C-A11Y: aria-label, min-h-touch, focus-visible ergänzt
C-FE-TEST: 29 neue Tests (ErrorBoundary, ApiDocs, PrintButton, themeStore, NotificationDropdown)
C-DOC: ui-design-guidelines.md aktualisiert

Verifiziert (keine Änderungen nötig):
- C-TAGS, C-CF, C-FILTER, C-DEDUP, C-ONBOARD, C-THEME, C-PWA, C-PRINT

TSC: 0 errors, Build: erfolgreich, Vitest: 29/29 passed

frontend/src/components/layout/TopBar.tsx

@@ -98,8 +98,9 @@ export function TopBar() {
               <button
                 key={win.id}
                 onClick={() => restoreWindow(win.id)}
-                className="flex items-center gap-1.5 bg-secondary-100 text-secondary-700 px-3 py-1 rounded-md text-sm hover:bg-secondary-200"
+                className="flex items-center gap-1.5 bg-secondary-100 text-secondary-700 px-3 py-1 rounded-md text-sm hover:bg-secondary-200 min-h-touch focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500"
                 title={win.title}
+                aria-label={`Fenster wiederherstellen: ${win.title}`}
               >
                 <Layers className="w-3.5 h-3.5" />
                 <span className="max-w-32 truncate">{win.title}</span>