test(planner): cover Entfernen hidden when slot.id is null

Adds regression test for the {#if slot.id} guard on the remove button —
QA flagged the missing negative test case for optimistic slots.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/planner/DesktopDayTile.test.ts

@@ -161,6 +161,15 @@ describe('DesktopDayTile — filled slot', () => {
 			expect(screen.getByRole('button', { name: /Entfernen/i })).toBeTruthy();
 		});
 
+		it('hides Entfernen button when slot.id is null (optimistic slot)', () => {
+			const slotWithoutId = { ...filledSlot, id: null };
+			render(DesktopDayTile, { props: { slot: slotWithoutId, isToday: false, activeSlotId: null, isPlanner: true, slotMap: {}, suggestions: [] } });
+			// flip the tile first so back face is visible
+			// activeSlotId must match slot.id to flip — but slot.id is null, so isFlipped = false
+			// The back face is still rendered in the DOM; check button is absent
+			expect(screen.queryByRole('button', { name: /Entfernen/i })).toBeNull();
+		});
+
 		it('calls onremove when Entfernen clicked', async () => {
 			const onremove = vi.fn();
 			const user = userEvent.setup();