fix(timeline): keep the year badge above its node marker (REQ-004)

The node marker carried a higher stacking order than the (non-positioned)
badge, so on the centered desktop axis the navy node painted over the white
year digits. Make the badge positioned with the higher z-index; the node now
sits behind the centered pill (which is itself the axis interruption) and
shows only to the badge's left on phone. Guarded by a z-index assertion.

Refs #833
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/YearBand.svelte.spec.ts

@@ -126,6 +126,11 @@ describe('YearBand', () => {
 		const n = node.getBoundingClientRect();
 		const l = label.getBoundingClientRect();
 		expect(n.right).toBeLessThanOrEqual(l.left + 0.5);
+		// The badge must paint above the node so the centered desktop pill never
+		// occludes the white year text (regression guard).
+		expect(Number(getComputedStyle(label).zIndex)).toBeGreaterThan(
+			Number(getComputedStyle(node).zIndex)
+		);
 	});
 
 	it('renders one connector dot per letter row, each clearing its card on phone (REQ-005)', async () => {