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.spec.ts

@@ -29,15 +29,16 @@ const pageData = (timeline: ReturnType<typeof makeTimelineDTO>) => ({
 });
 
 describe('/zeitstrahl page', () => {
-	it('wraps the timeline in a bordered, rounded canvas frame (REQ-001)', () => {
+	it('wraps the timeline in a padded canvas surface, without an outer border (REQ-001)', () => {
 		render(Page, {
 			data: pageData(makeTimelineDTO({ years: [makeYear(1909, [makeEntry()])] }))
 		});
 		const canvas = document.querySelector('[data-testid="timeline-canvas"]');
 		expect(canvas).not.toBeNull();
 		expect(canvas?.classList.contains('bg-canvas')).toBe(true);
-		expect(canvas?.classList.contains('border')).toBe(true);
-		// the timeline renders inside the frame
+		// The outer border was dropped in review (the page is already bg-canvas).
+		expect(canvas?.classList.contains('border')).toBe(false);
+		// the timeline renders inside the surface
 		expect(canvas?.querySelector('ol')).not.toBeNull();
 	});