feat(timeline): add /zeitstrahl route, SSR load, and nav link

SSR-first load fetches GET /api/timeline via createApiClient (auth cookie
forwarded), no query params for the global view (REQ-001), returns { timeline }
with no client-side fetch (REQ-002); 401 -> /login, any other non-ok ->
error(status, getErrorMessage(...)), never raw JSON, no PII logged (REQ-022).
The page renders <TimelineView> under the layout's <main>. Adds the Zeitstrahl
nav link (desktop + mobile) and 'timeline' to the eslint routes boundary
allow-list so the route may import the domain.

Refs #779
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/eslint.config.js

@@ -215,6 +215,7 @@ export default defineConfig(
 										'ocr',
 										'activity',
 										'conversation',
+										'timeline',
 										'shared'
 									]
 								}