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/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>