refactor(documents): extract Y-axis and X-axis components (#385)

Felix's review named "TimelineAxes" as one of four split targets.
The Y-axis and X-axis don't sit adjacent in the DOM — Y is a flex
sibling of the bars+X column — so two single-purpose components
beats a discriminator-prop component. tickIndicesFor and the
omitTickYear derivation move to TimelineXAxis where they belong.
Closes part 2 of Felix's component-split concern.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/document/TimelineDensityFilter.svelte

@@ -6,11 +6,12 @@ import {
 	clipBucketsToRange,
 	selectionBoundaryFrom,
 	selectionBoundaryTo,
-	tickIndicesFor,
 	formatTickLabel
 } from '$lib/document/timeline';
 import { getLocale } from '$lib/paraglide/runtime';
 import TimelineBars from '$lib/document/TimelineBars.svelte';
+import TimelineYAxis from '$lib/document/TimelineYAxis.svelte';
+import TimelineXAxis from '$lib/document/TimelineXAxis.svelte';
 import type { components } from '$lib/generated/api';
 
 type MonthBucket = components['schemas']['MonthBucket'];
@@ -219,8 +220,6 @@ const dragWindowRightPct = $derived.by(() => {
 	return ((filled.length - dragHighIndex - 1) / filled.length) * 100;
 });
 
-const tickIndices = $derived(tickIndicesFor(filled));
-
 // While dragging, expose the live preview range to assistive tech via a
 // polite live region. Empty text outside drag avoids announcing residual state.
 const dragLiveMessage = $derived.by(() => {
@@ -233,11 +232,6 @@ const dragLiveMessage = $derived.by(() => {
 		to: formatTickLabel(toLabel, getLocale())
 	});
 });
-const omitTickYear = $derived.by(() => {
-	if (filled.length === 0 || filled[0].month.length === 4) return false;
-	const firstYear = filled[0].month.slice(0, 4);
-	return filled.every((b) => b.month.slice(0, 4) === firstYear);
-});
 </script>
 
 {#if density !== null}
@@ -248,15 +242,7 @@ const omitTickYear = $derived.by(() => {
 		class="relative rounded-sm border border-line bg-surface px-3 pt-3 pb-2 shadow-sm"
 	>
 		<div class="flex">
-			<div
-				class="flex flex-col justify-between pr-1.5 text-right font-sans text-xs leading-none text-ink-3"
-				style="height: {BAR_AREA_HEIGHT}px;"
-				aria-hidden="true"
-				data-testid="timeline-y-axis"
-			>
-				<span>{maxCount}</span>
-				<span>0</span>
-			</div>
+			<TimelineYAxis maxCount={maxCount} barAreaHeight={BAR_AREA_HEIGHT} />
 
 			<div class="flex-1">
 				<TimelineBars
@@ -274,22 +260,7 @@ const omitTickYear = $derived.by(() => {
 					onbarclick={handleClick}
 				/>
 
-				<div
-					class="relative mt-1 h-4 font-sans text-xs leading-none text-ink-3"
-					aria-hidden="true"
-					data-testid="timeline-x-axis"
-				>
-					{#each tickIndices as idx (filled[idx]?.month)}
-						{@const tickLeftPct = ((idx + 0.5) / filled.length) * 100}
-						<span
-							class="absolute -translate-x-1/2 whitespace-nowrap"
-							data-testid="timeline-x-tick"
-							style="left: {tickLeftPct}%;"
-						>
-							{formatTickLabel(filled[idx].month, getLocale(), omitTickYear)}
-						</span>
-					{/each}
-				</div>
+				<TimelineXAxis filled={filled} />
 			</div>
 		</div>
 

frontend/src/lib/document/TimelineXAxis.svelte

@@ -0,0 +1,40 @@
+<script lang="ts">
+import { formatTickLabel, tickIndicesFor } from '$lib/document/timeline';
+import { getLocale } from '$lib/paraglide/runtime';
+import type { components } from '$lib/generated/api';
+
+type MonthBucket = components['schemas']['MonthBucket'];
+
+let {
+	filled
+}: {
+	filled: MonthBucket[];
+} = $props();
+
+const tickIndices = $derived(tickIndicesFor(filled));
+
+// When all visible buckets share a year, the X-axis omits the year so a
+// 12-month zoom reads as "Jan Feb Mär…" without repetition.
+const omitTickYear = $derived.by(() => {
+	if (filled.length === 0 || filled[0].month.length === 4) return false;
+	const firstYear = filled[0].month.slice(0, 4);
+	return filled.every((b) => b.month.slice(0, 4) === firstYear);
+});
+</script>
+
+<div
+	class="relative mt-1 h-4 font-sans text-xs leading-none text-ink-3"
+	aria-hidden="true"
+	data-testid="timeline-x-axis"
+>
+	{#each tickIndices as idx (filled[idx]?.month)}
+		{@const tickLeftPct = ((idx + 0.5) / filled.length) * 100}
+		<span
+			class="absolute -translate-x-1/2 whitespace-nowrap"
+			data-testid="timeline-x-tick"
+			style="left: {tickLeftPct}%;"
+		>
+			{formatTickLabel(filled[idx].month, getLocale(), omitTickYear)}
+		</span>
+	{/each}
+</div>

frontend/src/lib/document/TimelineYAxis.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+let {
+	maxCount,
+	barAreaHeight
+}: {
+	maxCount: number;
+	barAreaHeight: number;
+} = $props();
+</script>
+
+<div
+	class="flex flex-col justify-between pr-1.5 text-right font-sans text-xs leading-none text-ink-3"
+	style="height: {barAreaHeight}px;"
+	aria-hidden="true"
+	data-testid="timeline-y-axis"
+>
+	<span>{maxCount}</span>
+	<span>0</span>
+</div>