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' },