Feat: Add tag browser page at /tags with count-sorted tag list and library deep-link

- Extends GET /api/v1/tags with sort=count_desc and min_count query params
- New TagsComponent at /tags (public, no auth guard) shows all tags sorted by image count
- Clicking a tag navigates to /?tags=<name> for a pre-filtered library view
- LibraryComponent reads ?tags= query param on init to support deep-linking from tag browser
- Library header gains a "Browse tags" link to /tags for discoverability
- All 15 TDD tasks complete; ruff, ng lint, and ng build clean

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

api/tests/integration/test_search.py

@@ -3,6 +3,7 @@ T041 — GET /api/v1/images?tags=cat,funny → only images with both tags
 T042 — same query excludes images with only one matching tag
 """
 import io
+
 import pytest