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>

.specify/feature.json

@@ -1 +1 @@
-{"feature_directory": "specs/005-ui-polish"}
+{"feature_directory":"specs/006-header-nav-signout"}