refactor(timeline): drop the canvas outer border (REQ-001)

The page is already bg-canvas, so the frame's border was the only thing
making it visible; per review it reads cleaner without it. Keep the padded
bg-canvas surface; the timeline sits on the page without a frame line.

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

frontend/src/routes/zeitstrahl/+page.svelte

@@ -30,9 +30,10 @@ const metaLine = $derived.by(() => {
 </svelte:head>
 
 <div class="mx-auto max-w-5xl px-4 py-8">
-	<!-- The .tl-canvas sheet: a framed surface the timeline reads as a finished
-	     life-thread rather than bare page chrome (REQ-001). -->
-	<div data-testid="timeline-canvas" class="rounded-[10px] border border-line bg-canvas p-6">
+	<!-- The .tl-canvas sheet: a padded canvas surface for the timeline. The outer
+	     border is intentionally omitted (the page is already bg-canvas), per the
+	     review of REQ-001 — the sheet reads through its padding, not a frame line. -->
+	<div data-testid="timeline-canvas" class="rounded-[10px] bg-canvas p-6">
 		<h1 class="font-serif text-2xl font-bold text-brand-navy">{m.timeline_heading()}</h1>
 		{#if hasContent}
 			<p data-testid="timeline-meta" class="mt-1 mb-6 font-sans text-xs text-ink-3">{metaLine}</p>