feat(stammbaum): keyboard pan/zoom on the canvas (#692)

+/- zoom by the fixed step and arrow keys pan by a tenth of the visible
extent, emitted via onPanZoom. Provides the keyboard-only alternative path
required by NFR-A11Y-002. Nodes keep their own Enter/Space selection.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/StammbaumTree.svelte.test.ts

@@ -509,6 +509,45 @@ describe('StammbaumTree node rendering branches', () => {
 		node.dispatchEvent(new KeyboardEvent('keydown', { key: ' ', bubbles: true }));
 		expect(onSelect).toHaveBeenCalledWith(ID_A);
 	});
+});
+
+describe('StammbaumTree keyboard pan/zoom (#692)', () => {
+	const renderTree = (onPanZoom: ReturnType<typeof vi.fn>) =>
+		render(StammbaumTree, {
+			nodes: [{ id: ID_A, displayName: 'Anna', familyMember: true }],
+			edges: [],
+			selectedId: null,
+			panZoom: { x: 0, y: 0, z: 1 },
+			onPanZoom,
+			onSelect: () => {}
+		});
+
+	it('zooms in on "+" and out on "-" by the keyboard step (OQ-002)', async () => {
+		const onPanZoom = vi.fn();
+		renderTree(onPanZoom);
+		const svg = document.querySelector('svg')!;
+
+		svg.dispatchEvent(new KeyboardEvent('keydown', { key: '+', bubbles: true }));
+		expect(onPanZoom).toHaveBeenCalledTimes(1);
+		expect(onPanZoom.mock.calls[0][0].z).toBeCloseTo(1.1, 6);
+
+		onPanZoom.mockClear();
+		svg.dispatchEvent(new KeyboardEvent('keydown', { key: '-', bubbles: true }));
+		expect(onPanZoom.mock.calls[0][0].z).toBeCloseTo(0.9, 6);
+	});
+
+	it('pans right/down on arrow keys (REQ-PAN-004)', async () => {
+		const onPanZoom = vi.fn();
+		renderTree(onPanZoom);
+		const svg = document.querySelector('svg')!;
+
+		svg.dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowRight', bubbles: true }));
+		expect(onPanZoom.mock.calls[0][0].x).toBeGreaterThan(0);
+
+		onPanZoom.mockClear();
+		svg.dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowDown', bubbles: true }));
+		expect(onPanZoom.mock.calls[0][0].y).toBeGreaterThan(0);
+	});
 
 	it('does not call onSelect for other keys', async () => {
 		const onSelect = vi.fn();