T08b: Frontend Calendar UI (month view, kanban, ICS, resources, sharing)

- 8 calendar components (MonthView, KanbanBoard, AppointmentModal, TaskDetailPanel, IcsControls, ResourceBooking, SharingSettings + API client)

- 2 pages (/calendar, /calendar/kanban) + zustand store

- 17 vitest tests (MonthView 5, KanbanBoard 6, AppointmentModal 6) all passing

- i18n: calendar namespace in en/de (104 lines each, +exportSuccess key)

- TS strict mode pass, npm run build pass

frontend/src/stores/calendarStore.ts

@@ -0,0 +1,56 @@
+/**
+ * Calendar plugin UI store (zustand).
+ *
+ * Holds the currently selected month/week offset, the list of calendars the
+ * user owns / can write to, and the active calendar selection used by the
+ * Month view, Kanban view and ICS controls.
+ *
+ * Server state (entries, kanban board) lives in the API hooks layer — this
+ * store only carries view/UI selection state.
+ */
+
+import { create } from 'zustand';
+import type { Calendar } from '@/api/calendar';
+
+export interface CalendarState {
+  /** First day of the visible month (UTC midnight). */
+  visibleMonth: Date;
+  /** Selected calendar id (null = first available). */
+  activeCalendarId: string | null;
+  /** Cached calendar list (filled by fetchCalendars()). */
+  calendars: Calendar[];
+
+  setVisibleMonth: (date: Date) => void;
+  goToNextMonth: () => void;
+  goToPrevMonth: () => void;
+  goToToday: () => void;
+  setActiveCalendarId: (id: string | null) => void;
+  setCalendars: (calendars: Calendar[]) => void;
+}
+
+function startOfMonthUtc(d: Date): Date {
+  return new Date(Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), 1));
+}
+
+export const useCalendarStore = create<CalendarState>((set) => ({
+  visibleMonth: startOfMonthUtc(new Date()),
+  activeCalendarId: null,
+  calendars: [],
+
+  setVisibleMonth: (date) => set({ visibleMonth: startOfMonthUtc(date) }),
+  goToNextMonth: () =>
+    set((s) => {
+      const next = new Date(s.visibleMonth);
+      next.setUTCMonth(next.getUTCMonth() + 1);
+      return { visibleMonth: next };
+    }),
+  goToPrevMonth: () =>
+    set((s) => {
+      const next = new Date(s.visibleMonth);
+      next.setUTCMonth(next.getUTCMonth() - 1);
+      return { visibleMonth: next };
+    }),
+  goToToday: () => set({ visibleMonth: startOfMonthUtc(new Date()) }),
+  setActiveCalendarId: (id) => set({ activeCalendarId: id }),
+  setCalendars: (calendars) => set({ calendars }),
+}));