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/app/routers/tags.py

@@ -12,9 +12,13 @@ async def list_tags(
     q: str | None = None,
     limit: int = 100,
     offset: int = 0,
+    sort: str = "name",
+    min_count: int = 0,
     db: AsyncSession = Depends(get_db),
 ):
-    limit = min(limit, 200)
+    limit = min(limit, 500)
     tag_repo = TagRepository(db)
-    items, total = await tag_repo.list_tags(prefix=q, limit=limit, offset=offset)
+    items, total = await tag_repo.list_tags(
+        prefix=q, limit=limit, offset=offset, sort=sort, min_count=min_count
+    )
     return {"items": items, "total": total, "limit": limit, "offset": offset}