feat(M3): Dashboard-Builder — Edit-Modus, Drag&Drop, Palette, Tabs (#361)

- DashboardBuilder: @dnd-kit 12-Spalten-Flow-Grid (seed-konsistent), View/Edit-Schalter, Resize, Tab-Verwaltung, Dashboard-CRUD + Set-Default, Dirty-Save
- MiniAppHost ersetzt DashboardWidgetLoader (lazy Registry + settings-Props); Palette nur renderbare Apps (component-Filter)
- WidgetSettingsForm generisch aus settings_schema; Bestands-Widgets settings-fähig (RecentContacts: limit)
- api/miniapps.ts + api/dashboards.ts (TanStack-Query-Hooks, documents.ts-Muster)
- Dashboard.tsx = Builder-Host (StatCards/SystemMetrics bleiben bis M4); Legacy-Grid/Loader gelöscht, Geister-Test ersetzt
- Tests: Builder 13/13, Page 11/11, i18n de/en, tsc clean, build OK

frontend/src/api/dashboards.ts

@@ -0,0 +1,97 @@
+/**
+ * Personal dashboards API hooks (Phase M2/M3).
+ *
+ * Layout shape (validated server-side, DashboardLayout):
+ * { version: 1, tabs: [{ id, name, widgets: [{ app_id, settings, col, row, col_span, row_span }] }] }
+ */
+
+import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
+import { apiGet, apiPost, apiPut, apiDelete } from './client';
+
+export interface DashboardWidget {
+  app_id: string;
+  settings: Record<string, unknown>;
+  col: number;
+  row: number;
+  col_span: number;
+  row_span: number;
+}
+
+export interface DashboardTab {
+  id: string;
+  name: string;
+  widgets: DashboardWidget[];
+}
+
+export interface DashboardLayout {
+  version: number;
+  tabs: DashboardTab[];
+}
+
+export interface Dashboard {
+  id: string;
+  name: string;
+  layout: DashboardLayout;
+  is_default: boolean;
+  user_id: string;
+  created_at: string | null;
+  updated_at: string | null;
+}
+
+export interface DashboardInput {
+  name: string;
+}
+
+export interface DashboardUpdateInput {
+  name?: string;
+  layout?: DashboardLayout;
+}
+
+export const DASHBOARD_QUERY_KEYS = {
+  all: ['dashboards'] as const,
+  one: (id: string) => ['dashboards', id] as const,
+};
+
+export function useDashboards() {
+  return useQuery({
+    queryKey: DASHBOARD_QUERY_KEYS.all,
+    queryFn: () => apiGet<Dashboard[]>('/dashboards'),
+    staleTime: 30 * 1000,
+  });
+}
+
+export function useCreateDashboard() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (input: DashboardInput) => apiPost<Dashboard>('/dashboards', input),
+    onSuccess: () => qc.invalidateQueries({ queryKey: DASHBOARD_QUERY_KEYS.all }),
+  });
+}
+
+export function useUpdateDashboard() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: ({ id, input }: { id: string; input: DashboardUpdateInput }) =>
+      apiPut<Dashboard>(`/dashboards/${id}`, input),
+    onSuccess: (_data, vars) => {
+      qc.invalidateQueries({ queryKey: DASHBOARD_QUERY_KEYS.all });
+      qc.invalidateQueries({ queryKey: DASHBOARD_QUERY_KEYS.one(vars.id) });
+    },
+  });
+}
+
+export function useDeleteDashboard() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (id: string) => apiDelete<void>(`/dashboards/${id}`),
+    onSuccess: () => qc.invalidateQueries({ queryKey: DASHBOARD_QUERY_KEYS.all }),
+  });
+}
+
+export function useSetDefaultDashboard() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (id: string) => apiPost<Dashboard>(`/dashboards/${id}/set-default`),
+    onSuccess: () => qc.invalidateQueries({ queryKey: DASHBOARD_QUERY_KEYS.all }),
+  });
+}