Feat: Header title links to grid; sign-out redirects to grid

Make the app title a clickable link to / so users can return to the
image grid from any sub-page without the browser back button. Change
the sign-out destination from /login to / since the grid is publicly
accessible and avoids unnecessary friction post-logout.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

specs/006-header-nav-signout/tasks.md

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+# Tasks: Header Navigation & Sign-Out Destination
+
+## Phase 1: Tests (TDD — write first)
+
+- [X] T001 Add component tests for header title routerLink to `/` and sign-out navigation to `/` in `ui/src/app/app.component.spec.ts`
+
+## Phase 2: Implementation
+
+- [X] T002 Wrap the `.app-name` span in a `routerLink="/"` anchor in `ui/src/app/app.component.ts`
+- [X] T003 Change `onLogout()` navigation target from `/login` to `/` in `ui/src/app/app.component.ts`
+
+## Phase 3: Validation
+
+- [X] T004 Run `ng lint` in `ui/` — zero violations
+- [X] T005 Run `ng build` in `ui/` — zero errors