refactor(timeline): single-source the spine X position via --spine-x

The spine offset (0.5rem phone / 50% desktop) was hard-coded in both
TimelineView's .timeline-axis::before and YearBand's .year-node/.letter-dot,
kept in sync only by a comment. Declare --spine-x once on .timeline-axis
and have the markers consume it by inheritance, so a change to the spine
position moves the markers with it. Add a test that the year-node tracks
the token.

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

frontend/src/lib/timeline/TimelineView.svelte

@@ -92,8 +92,12 @@ const isEmpty = $derived(timeline.years.length === 0 && timeline.undated.length
 <style>
 /* Establish a stacking context so the spine (z-index: -1) sits behind the
 	   in-flow cards/pills/strips but still in front of the canvas — the line is
-	   always background; the badges, dots and markers ride on top of it. */
+	   always background; the badges, dots and markers ride on top of it.
+	   --spine-x is the single source of truth for the spine's X position; the
+	   year-node and connector dots in YearBand consume it via inheritance so the
+	   markers can never desync from the line. */
 .timeline-axis {
+	--spine-x: 0.5rem;
 	position: relative;
 	z-index: 0;
 }
@@ -106,7 +110,7 @@ const isEmpty = $derived(timeline.years.length === 0 && timeline.undated.length
 	position: absolute;
 	top: 0;
 	bottom: 0;
-	left: 0.5rem;
+	left: var(--spine-x);
 	width: 2px;
 	z-index: -1;
 	/* Three-stop life-thread: mint → navy → slate. Slate lives only as
@@ -115,8 +119,10 @@ const isEmpty = $derived(timeline.years.length === 0 && timeline.undated.length
 }
 
 @media (min-width: 1024px) {
+	.timeline-axis {
+		--spine-x: 50%;
+	}
 	.timeline-axis::before {
-		left: 50%;
 		transform: translateX(-50%);
 	}
 }

frontend/src/lib/timeline/YearBand.svelte

@@ -85,9 +85,10 @@ const rows = $derived.by<Row[]>(() => {
 	z-index: 1;
 }
 
-/* Spine geometry below mirrors TimelineView's .timeline-axis::before
-	   (left: 0.5rem phone / left: 50% desktop) — keep the two in sync so the
-	   markers never desync from the spine. #833 REQ-003/004/005. */
+/* Markers ride on the spine's single source of truth: --spine-x, declared
+	   once on TimelineView's .timeline-axis and inherited here (0.5rem phone /
+	   50% desktop). The 0.5rem fallback only applies when a YearBand is rendered
+	   outside the axis (e.g. component tests). #833 REQ-003/004/005. */
 
 /* Phone (< 1024px): badge sits at the left spine, clearing the node marker.
 	   The badge sits above the node (z-index) so on desktop, where the centered
@@ -105,7 +106,7 @@ const rows = $derived.by<Row[]>(() => {
 .year-node {
 	position: absolute;
 	top: 50%;
-	left: 0.5rem;
+	left: var(--spine-x, 0.5rem);
 	width: 11px;
 	height: 11px;
 	border-radius: 9999px;
@@ -121,7 +122,7 @@ const rows = $derived.by<Row[]>(() => {
 .letter-dot {
 	position: absolute;
 	top: 0.9rem;
-	left: 0.5rem;
+	left: var(--spine-x, 0.5rem);
 	width: 13px;
 	height: 13px;
 	border-radius: 9999px;
@@ -140,9 +141,8 @@ const rows = $derived.by<Row[]>(() => {
 		margin-left: auto;
 		margin-right: auto;
 	}
-	.year-node {
-		left: 50%;
-	}
+	/* .year-node needs no desktop override — it inherits --spine-x: 50% from
+		   the axis. */
 	.letter-row {
 		width: 50%;
 		padding-left: 0;

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

@@ -145,4 +145,23 @@ describe('YearBand', () => {
 			card.getBoundingClientRect().left + 0.5
 		);
 	});
+
+	it('positions the year-node from the inherited --spine-x token (REQ-003/004)', async () => {
+		await page.viewport(375, 800);
+		// The spine X lives once on .timeline-axis as --spine-x; the markers must
+		// track that token, not a hard-coded offset, so they never desync.
+		document.documentElement.style.setProperty('--spine-x', '3rem');
+		try {
+			render(YearBand, { year: makeYear(1914, [makeEntry()]) });
+			const node = document.querySelector('[data-testid="year-node"]') as HTMLElement;
+			const section = document.querySelector('section') as HTMLElement;
+			const n = node.getBoundingClientRect();
+			const s = section.getBoundingClientRect();
+			const nodeCenter = n.left + n.width / 2;
+			// --spine-x:3rem = 48px from the band's left edge
+			expect(Math.abs(nodeCenter - s.left - 48)).toBeLessThan(2);
+		} finally {
+			document.documentElement.style.removeProperty('--spine-x');
+		}
+	});
 });