feat(timeline): add precision-aware date-label facade

timelineDateLabel delegates to the shared formatDocumentDate so a timeline
chip renders identically to the same date on a document, in the active
locale (REQ-001/REQ-002). UNKNOWN precision and null/undefined/'' eventDate
short-circuit to null with no formatter call (REQ-003/REQ-004); raw is always
null since timeline events carry no verbatim spreadsheet cell. The facade
owns no precision logic of its own (REQ-005).

Register the new `timeline` frontend domain in the eslint boundaries config
(allowed to import only `shared`) and add src/lib/timeline/** to the vitest
coverage include (REQ-006). The spec partially mocks the paraglide runtime
via importOriginal so getLocale is stubbed while the formatter still resolves
real season/range message exports.

Refs #778
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/eslint.config.js

@@ -158,6 +158,7 @@ export default defineConfig(
 				{ type: 'ocr', pattern: 'src/lib/ocr/**' },
 				{ type: 'activity', pattern: 'src/lib/activity/**' },
 				{ type: 'conversation', pattern: 'src/lib/conversation/**' },
+				{ type: 'timeline', pattern: 'src/lib/timeline/**' },
 				{ type: 'shared', pattern: 'src/lib/shared/**' },
 				{ type: 'routes', pattern: 'src/routes/**' }
 			]
@@ -198,6 +199,7 @@ export default defineConfig(
 						{ from: { type: 'user' }, allow: { to: { type: ['shared'] } } },
 						{ from: { type: 'notification' }, allow: { to: { type: ['shared'] } } },
 						{ from: { type: 'conversation' }, allow: { to: { type: ['shared'] } } },
+						{ from: { type: 'timeline' }, allow: { to: { type: ['shared'] } } },
 						{ from: { type: 'shared' }, allow: { to: { type: ['shared'] } } },
 						{
 							from: { type: 'routes' },

frontend/src/lib/timeline/dateLabel.spec.ts

@@ -5,8 +5,13 @@ import { timelineDateLabel } from './dateLabel';
 
 // The mocked path MUST include the `.js` suffix to match the import in
 // dateLabel.ts — a specifier mismatch silently skips the mock and the helper
-// would resolve the real locale.
-vi.mock('$lib/paraglide/runtime.js', () => ({ getLocale: vi.fn(() => 'de') }));
+// would resolve the real locale. We partially mock via importOriginal so only
+// getLocale is stubbed: the shared formatter still pulls real Paraglide message
+// exports (season words, range prefixes) off the same runtime module.
+vi.mock('$lib/paraglide/runtime.js', async (importOriginal) => ({
+	...(await importOriginal<typeof import('$lib/paraglide/runtime.js')>()),
+	getLocale: vi.fn(() => 'de')
+}));
 
 describe('timelineDateLabel', () => {
 	beforeEach(() => {

frontend/src/lib/timeline/dateLabel.ts

@@ -0,0 +1,30 @@
+import { formatDocumentDate, type DatePrecision } from '$lib/shared/utils/documentDate';
+import { getLocale } from '$lib/paraglide/runtime.js';
+
+/**
+ * Renders a timeline event's date by delegating to the shared
+ * {@link formatDocumentDate} — so a timeline chip reads identically to the same
+ * date on a document, in whichever locale is active. This module is a thin
+ * façade: it owns NO precision or rendering logic (REQ-005), only the two
+ * timeline-specific decisions below.
+ *
+ * @param eventDate the event's anchor day (`YYYY-MM-DD`). `null`, `undefined`
+ *   and `''` are equivalent — all mean "undated" and yield `null` WITHOUT
+ *   calling the formatter (REQ-004).
+ * @param precision the event's precision metadata; `'UNKNOWN'` yields `null`
+ *   (no chip) even when a date is present (REQ-003).
+ * @param eventDateEnd the RANGE end day; `undefined` and `null` are equivalent
+ *   and both mean an open-ended range.
+ * @returns the localized label, or `null` for an UNKNOWN/undated event.
+ */
+export function timelineDateLabel(
+	eventDate: string | null | undefined,
+	precision: DatePrecision,
+	eventDateEnd?: string | null
+): string | null {
+	if (precision === 'UNKNOWN' || !eventDate) return null;
+	// raw is always null for timeline events — there is no verbatim spreadsheet
+	// cell to interpolate; season words derive from the structured anchor month
+	// (never from untrusted text). See documentDate.ts for the raw contract.
+	return formatDocumentDate(eventDate, precision, eventDateEnd ?? null, null, getLocale());
+}

frontend/vite.config.ts

@@ -72,6 +72,7 @@ export default defineConfig({
 				'src/lib/shared/server/**',
 				'src/lib/shared/discussion/**',
 				'src/lib/document/**',
+				'src/lib/timeline/**',
 				'src/hooks.server.ts'
 			],
 			exclude: [