fix(dates): use AdminDatePicker everywhere + mode-aware default placeholder

- The project create-modal start/end dates used raw <input type="date">;
  converted to <AdminDatePicker> so every date field in the app now goes
  through the shared component (react-datepicker on desktop, native on touch).
- AdminDatePicker now defaults its placeholder by mode (date 'dd.mm.rrrr',
  month 'mm.rrrr', time 'hh:mm') so empty date fields always show a format
  hint; explicit placeholders (e.g. 'Od'/'Do' range filters) still override.
- Dropped WarehouseReceiptForm's ad-hoc 'dd.mm.yyyy' so it inherits the
  unified Czech default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/pages/WarehouseReceiptForm.tsx

@@ -378,7 +378,6 @@ export default function WarehouseReceiptForm() {
                       delivery_note_date: val,
                     }))
                   }
-                  placeholder="dd.mm.yyyy"
                 />
               </FormField>
             </div>