fix(admin): stop the event-date field crashing the page on backspace

Repro: create an event, click into the date field, backspace a day digit.
The whole page white-screened and needed a reload.

Root cause: LocalizedDateInput's `toIso` only checked the day/month were
1-2 digits, not that they formed a real date — so a mid-backspace value
like "0/07/2026" was coerced to the string "2026-07-00" and committed to
`event_date`. CreateEventPage then rendered
`format(addDays(new Date('2026-07-00'), days))`, and date-fns `format`
throws RangeError on an Invalid Date — thrown during render, so React
tore the tree down to the error boundary.

Two complementary fixes:
- `toIso` round-trips the parsed y/m/d through `Date` and rejects
  impossible dates (day 00, month 13, 31 Feb…), so the field never
  commits a value that isn't a real calendar date.
- `useLocalizedDate.format`/`formatDistanceToNow` guard with `isValid`
  and return '' instead of throwing — defence in depth for the ~57 call
  sites that could otherwise white-screen on a bad date.

Verified live: backspacing to a partial/invalid date no longer crashes
(the form stays rendered), a valid date still commits + the expiry
preview renders. Adds a LocalizedDateInput regression test; tsc + build
green.

frontend/src/components/common/LocalizedDateInput.tsx

@@ -90,6 +90,13 @@ export const LocalizedDateInput: React.FC<LocalizedDateInputProps> = ({
       [d, mo, y] = [a, b, c];
     }
     if (!/^\d{1,2}$/.test(d) || !/^\d{1,2}$/.test(mo) || !/^\d{4}$/.test(y)) return '';
+    // Reject impossible calendar dates (day 00, month 13, 31 Feb…) — a partial
+    // value mid-backspace like "0/07/2026" is otherwise coerced to "2026-07-00",
+    // which is a valid string but an Invalid Date that crashes date-fns format()
+    // downstream. Round-trip through Date to confirm the components survive.
+    const yy = Number(y), mm = Number(mo), dd = Number(d);
+    const probe = new Date(yy, mm - 1, dd);
+    if (probe.getFullYear() !== yy || probe.getMonth() !== mm - 1 || probe.getDate() !== dd) return '';
     return `${y}-${mo.padStart(2, '0')}-${d.padStart(2, '0')}`;
   };
 

frontend/src/components/common/__tests__/LocalizedDateInput.test.tsx

@@ -0,0 +1,39 @@
+import React from 'react';
+import { render, screen, fireEvent } from '@testing-library/react';
+import { vi } from 'vitest';
+import { LocalizedDateInput } from '../LocalizedDateInput';
+
+// Stub the settings hook so the component doesn't need a QueryClient; falls
+// back to the default DD.MM.YYYY display format. The parse/validation under
+// test is separator-independent.
+vi.mock('../../../hooks/usePublicSettings', () => ({
+  usePublicSettings: () => ({ settings: {} }),
+}));
+
+describe('LocalizedDateInput', () => {
+  const renderInput = (value = '2026-07-07') => {
+    const onChange = vi.fn();
+    render(<LocalizedDateInput value={value} onChange={onChange} />);
+    const input = screen.getByDisplayValue('07.07.2026') as HTMLInputElement;
+    return { input, onChange };
+  };
+
+  it('does not commit an impossible date mid-edit (regression: backspacing the day → "2026-07-00" crashed the page)', () => {
+    const { input, onChange } = renderInput();
+    // Backspacing a day digit leaves "0.07.2026" — a syntactically complete but
+    // invalid date. It must NOT propagate (used to coerce to "2026-07-00",
+    // which crashed date-fns format() downstream).
+    fireEvent.change(input, { target: { value: '0.07.2026' } });
+    expect(onChange).not.toHaveBeenCalled();
+
+    // Nor may an out-of-range calendar date (31 Feb).
+    fireEvent.change(input, { target: { value: '31.02.2026' } });
+    expect(onChange).not.toHaveBeenCalled();
+  });
+
+  it('commits a complete, valid date as ISO', () => {
+    const { input, onChange } = renderInput();
+    fireEvent.change(input, { target: { value: '15.08.2026' } });
+    expect(onChange).toHaveBeenCalledWith('2026-08-15');
+  });
+});