feat(timeline): add the tinted Thema bucket-header chip

A fully-tinted root-tag chip for Thema-mode bucket headers (#827, REQ-015):
fill and label both derive from the tag's --c-tag-* token via a color-mix wash
so the label keeps ≥4.5:1 contrast in light and dark mode. A null or unknown
token falls back to a neutral chip with no broken colour. Curator text is
{...}-escaped (REQ-009). Distinct from the neutral per-letter TagChip.

Refs #827
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/BucketHeaderChip.svelte

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+<script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+
+/**
+ * The header chip of a Thema-mode root-tag bucket (#827, REQ-015): a *fully-tinted* chip whose
+ * fill and label both derive from the root tag's `--c-tag-*` colour token — distinct from the
+ * neutral per-letter {@link TagChip} (a surface pill with a tiny colour square). The label uses
+ * the saturated token as text over a subtle `color-mix` wash of the same token, so the ≥4.5:1
+ * label contrast holds in both light and dark themes. A `null` colour — or any value outside the
+ * known token set (the §2 `krieg`/`weih`/`fam` are demo class names, not tokens) — falls back to a
+ * neutral chip with no `var(--c-tag-)` reference, never a broken colour. The name is
+ * curator/import-derived and rendered through default `{...}` escaping, never `{@html}` (REQ-009).
+ */
+const TAG_COLORS = new Set([
+	'sage',
+	'sienna',
+	'amber',
+	'slate',
+	'violet',
+	'rose',
+	'cobalt',
+	'moss',
+	'sand',
+	'coral'
+]);
+
+let { name, color }: { name: string; color: string | null } = $props();
+
+const token = $derived(color && TAG_COLORS.has(color) ? color : null);
+const chipStyle = $derived(
+	token
+		? `background-color: color-mix(in srgb, var(--c-tag-${token}) 14%, transparent); color: var(--c-tag-${token})`
+		: ''
+);
+const dotStyle = $derived(token ? `background-color: var(--c-tag-${token})` : '');
+</script>
+
+<span
+	data-testid="bucket-header-chip"
+	title={name}
+	style={chipStyle}
+	class="inline-flex max-w-full items-center gap-1.5 rounded-full px-2.5 py-0.5 font-sans text-xs font-semibold"
+	class:border={!token}
+	class:border-line={!token}
+	class:bg-surface={!token}
+	class:text-ink-3={!token}
+>
+	<span class="sr-only">{m.timeline_tag_chip_label()}: </span>
+	<span
+		data-testid="bucket-header-chip-dot"
+		aria-hidden="true"
+		style={dotStyle}
+		class="inline-block h-2 w-2 flex-shrink-0 rounded-sm"
+		class:bg-ink-3={!token}
+	></span>
+	<span style="overflow: hidden; text-overflow: ellipsis; white-space: nowrap; min-width: 0"
+		>{name}</span
+	>
+</span>

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

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+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import * as m from '$lib/paraglide/messages.js';
+import BucketHeaderChip from './BucketHeaderChip.svelte';
+
+afterEach(() => cleanup());
+
+describe('BucketHeaderChip (REQ-015/009)', () => {
+	it('renders the root-tag name', () => {
+		render(BucketHeaderChip, { name: 'Krieg', color: 'sienna' });
+		expect(document.body.textContent).toContain('Krieg');
+	});
+
+	it('tints the chip with var(--c-tag-{token}) for a known colour token (REQ-015)', () => {
+		render(BucketHeaderChip, { name: 'Krieg', color: 'sienna' });
+		const chip = document.querySelector('[data-testid="bucket-header-chip"]') as HTMLElement;
+		expect(chip.getAttribute('style')).toContain('var(--c-tag-sienna)');
+	});
+
+	it('renders a neutral chip with no --c-tag- binding when colour is null (REQ-015)', () => {
+		render(BucketHeaderChip, { name: 'Ohne Thema', color: null });
+		expect(document.body.textContent).toContain('Ohne Thema');
+		expect(document.body.innerHTML).not.toContain('var(--c-tag-');
+	});
+
+	it('falls back to neutral for an unknown colour token, never a broken var (REQ-015)', () => {
+		// "krieg" is a §2 demo class name, not a real --c-tag-* token.
+		render(BucketHeaderChip, { name: 'Krieg', color: 'krieg' });
+		expect(document.body.innerHTML).not.toContain('var(--c-tag-');
+	});
+
+	it('prefixes the name with an sr-only theme label so colour is never the only cue', () => {
+		render(BucketHeaderChip, { name: 'Krieg', color: 'sienna' });
+		const srOnly = document.querySelector('.sr-only');
+		expect(srOnly?.textContent).toContain(m.timeline_tag_chip_label());
+	});
+
+	it('renders an HTML-bearing name as inert text, never markup (REQ-009)', () => {
+		const evil = '<img src=x onerror="alert(1)">';
+		render(BucketHeaderChip, { name: evil, color: null });
+		expect(document.body.textContent).toContain(evil);
+		expect(document.querySelector('img')).toBeNull();
+	});
+});