fix(calendar): don't put a fixed reference date in the month header

dayHeaderContent assumed arg.date is always the real column date. It is in
the time-grid views, but FullCalendar v6 fills it from an internal reference
week (1970-01-04..10) for dayGridMonth headers, so the month header read a
fixed "Mo 05.01. ... So 04.01." regardless of the visible month. Body dates
were correct; only the header row was wrong.

Interpretation (flagged as ambiguous): a month-view column header labels seven
generic weekday columns shared by every week in the grid -- it has no single
date, so forcing one in is wrong by construction rather than just mis-computed.
Month view now renders the localized weekday alone, which is also FC's own
default there; timeGridWeek keeps weekday + DD.MM. since each column really is
one date.

Branches on view.type === 'dayGridMonth' exactly, not a dayGrid prefix:
dayGridWeek/dayGridDay do have real per-column dates and a prefix match would
break them if either is ever added.

Extracted to an exported formatDayHeader so it is testable without mounting
the page; the test mounts a real FullCalendar in both views, so an upgrade
that changes the arg.date contract fails rather than silently regresses.
FullCalendar dependency untouched.

Refs testplan REPORT.md #10 (Part 8, S9).

frontend/src/pages/admin/clients/CalendarPage.tsx

@@ -74,6 +74,38 @@ const COLOR_HOURS_LOCKED = '#9CA3AF'; // gray-400 (greyed)
 const COLOR_QUOTE_BORDER = '#F59E0B'; // amber-500
 const COLOR_CONTRACT_BORDER = '#A855F7'; // purple-500
 
+/**
+ * Column-header text for the calendar's day headers.
+ *
+ * FC's default in en-US renders as "Thu 5/21" (M/D), which is wrong for
+ * DE / CH operators who expect day.month. dayHeaderContent (NOT
+ * dayHeaderFormat, which only accepts an Intl options object in FC v6 and
+ * throws if handed a function) returns the rendered string directly.
+ * Locale-aware short weekday + explicit DD.MM. to match the project-wide
+ * useLocalizedDate convention (per
+ * feedback_respect_general_format_settings.md).
+ *
+ * Only the time-grid views have one date per column. dayGridMonth's header
+ * row labels seven generic weekday columns shared by every week in the grid,
+ * and FC fills `arg.date` there from its internal reference week
+ * (1970-01-04..10) — which is why the month header used to read a fixed
+ * "Mo 05.01. … So 04.01." whatever month was on screen. The weekday is still
+ * correct, so month view renders the weekday alone: there is no single date
+ * that column could legitimately show.
+ */
+export function formatDayHeader(date: Date, viewType: string, language: string): string {
+  const weekday = date.toLocaleDateString(language || 'en', {
+    weekday: 'short',
+    timeZone: 'UTC',
+  });
+  if (viewType === 'dayGridMonth') {
+    return weekday;
+  }
+  const day = String(date.getUTCDate()).padStart(2, '0');
+  const month = String(date.getUTCMonth() + 1).padStart(2, '0');
+  return `${weekday} ${day}.${month}.`;
+}
+
 /**
  * Convert a backend CalendarItem to a FullCalendar EventInput. Embeds
  * the original item in `extendedProps` so the event-click handler can
@@ -486,24 +518,9 @@ export const CalendarPage: React.FC = () => {
           locale={i18n.language || 'en'}
           slotLabelFormat={fcTimeFormat}
           eventTimeFormat={fcTimeFormat}
-          // Week-view column headers. FC's default in en-US renders as
-          // "Thu 5/21" (M/D), which is wrong for DE / CH operators who
-          // expect day.month. dayHeaderContent (NOT dayHeaderFormat,
-          // which only accepts an Intl options object in FC v6 and
-          // throws if handed a function) returns the rendered string
-          // directly. Locale-aware short weekday + explicit DD.MM. to
-          // match the project-wide useLocalizedDate convention (per
-          // feedback_respect_general_format_settings.md).
-          dayHeaderContent={(arg) => {
-            const d = arg.date;
-            const day = String(d.getUTCDate()).padStart(2, '0');
-            const month = String(d.getUTCMonth() + 1).padStart(2, '0');
-            const weekday = d.toLocaleDateString(i18n.language || 'en', {
-              weekday: 'short',
-              timeZone: 'UTC',
-            });
-            return `${weekday} ${day}.${month}.`;
-          }}
+          // Column headers — see formatDayHeader above for the per-view
+          // semantics (month headers carry no date).
+          dayHeaderContent={(arg) => formatDayHeader(arg.date, arg.view.type, i18n.language)}
           headerToolbar={{
             left: 'prev,next today',
             center: 'title',

frontend/src/pages/admin/clients/__tests__/CalendarPage.dayHeader.test.tsx

@@ -0,0 +1,77 @@
+/**
+ * Calendar column headers per view (#S9).
+ *
+ * `dayHeaderContent` was written for the week view, where FullCalendar hands
+ * the callback the real date of that column. In dayGridMonth the header row
+ * labels seven generic weekday columns shared by every week, and FC fills
+ * `arg.date` from its internal reference week (1970-01-04..10) — so the month
+ * header read a fixed "Mo 05.01. … So 04.01." no matter which month was on
+ * screen, while the day cells underneath were correct.
+ *
+ * The second block below mounts a real FullCalendar to pin FC's actual arg
+ * behaviour: it is the library quirk, not our formatting, that these tests
+ * exist to catch if a future FC upgrade changes it.
+ */
+import { render, screen } from '@testing-library/react';
+import { describe, expect, it } from 'vitest';
+import FullCalendar from '@fullcalendar/react';
+import dayGridPlugin from '@fullcalendar/daygrid';
+import timeGridPlugin from '@fullcalendar/timegrid';
+
+import { formatDayHeader } from '../CalendarPage';
+
+describe('formatDayHeader', () => {
+  it('renders weekday + DD.MM. in the week view, where the column has a real date', () => {
+    const d = new Date(Date.UTC(2026, 8, 1)); // Tue 2026-09-01
+    expect(formatDayHeader(d, 'timeGridWeek', 'en')).toBe('Tue 01.09.');
+  });
+
+  it('renders the weekday alone in month view, where the column has no single date', () => {
+    // FC's month-view reference week — the date that used to leak into the UI.
+    const reference = new Date(Date.UTC(1970, 0, 5)); // Mon 1970-01-05
+    expect(formatDayHeader(reference, 'dayGridMonth', 'en')).toBe('Mon');
+    expect(formatDayHeader(reference, 'dayGridMonth', 'en')).not.toMatch(/\d/);
+  });
+
+  it('keeps the weekday locale-aware in both views', () => {
+    const d = new Date(Date.UTC(2026, 8, 1));
+    expect(formatDayHeader(d, 'dayGridMonth', 'de')).toBe('Di');
+    expect(formatDayHeader(d, 'timeGridWeek', 'de')).toBe('Di 01.09.');
+  });
+});
+
+describe('FullCalendar day headers', () => {
+  const headerTexts = () =>
+    Array.from(document.querySelectorAll('.fc-col-header-cell')).map(
+      (c) => c.textContent?.trim() ?? ''
+    );
+
+  const renderCalendar = (view: 'dayGridMonth' | 'timeGridWeek') =>
+    render(
+      <FullCalendar
+        plugins={[dayGridPlugin, timeGridPlugin]}
+        initialView={view}
+        initialDate="2026-09-15"
+        timeZone="UTC"
+        firstDay={1}
+        headerToolbar={false}
+        height="auto"
+        dayHeaderContent={(arg) => formatDayHeader(arg.date, arg.view.type, 'en')}
+      />
+    );
+
+  it('month view shows seven dateless weekday headers', () => {
+    renderCalendar('dayGridMonth');
+    expect(headerTexts()).toEqual(['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']);
+    // The old formatting leaked FC's 1970 reference week into the UI.
+    expect(screen.queryByText(/05\.01\./)).toBeNull();
+  });
+
+  it('week view still shows the real date of each column', () => {
+    renderCalendar('timeGridWeek');
+    expect(headerTexts()).toEqual([
+      'Mon 14.09.', 'Tue 15.09.', 'Wed 16.09.',
+      'Thu 17.09.', 'Fri 18.09.', 'Sat 19.09.', 'Sun 20.09.',
+    ]);
+  });
+});