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>

ui/src/app/library/library.component.spec.ts

@@ -1,5 +1,5 @@
 import { TestBed } from '@angular/core/testing';
-import { provideRouter } from '@angular/router';
+import { provideRouter, ActivatedRoute } from '@angular/router';
 import { provideHttpClient } from '@angular/common/http';
 import { provideHttpClientTesting } from '@angular/common/http/testing';
 import { of } from 'rxjs';
@@ -7,6 +7,16 @@ import { LibraryComponent } from './library.component';
 import { ImageService } from '../services/image.service';
 import { routes } from '../app.routes';
 
+function makeActivatedRoute(queryParams: Record<string, string> = {}) {
+  return {
+    snapshot: {
+      queryParamMap: {
+        get: (key: string) => queryParams[key] ?? null,
+      },
+    },
+  };
+}
+
 const EMPTY_PAGE = { items: [], total: 0, limit: 50, offset: 0 };
 const ONE_IMAGE = {
   items: [{ id: '1', filename: 'a.jpg', tags: ['cat'], hash: '', mime_type: 'image/jpeg', size_bytes: 1, width: 1, height: 1, storage_key: '', thumbnail_key: null, created_at: '' }],
@@ -107,4 +117,32 @@ describe('LibraryComponent', () => {
     fixture.componentInstance.onImgError(event);
     expect(imgEl.src).toBe(originalSrc);
   });
+
+  it('pre-populates activeFilters from ?tags= query param on init', () => {
+    TestBed.overrideProvider(ActivatedRoute, { useValue: makeActivatedRoute({ tags: 'cat,funny' }) });
+    const fixture = TestBed.createComponent(LibraryComponent);
+    const imgSvc = TestBed.inject(ImageService);
+    const listSpy = spyOn(imgSvc, 'list').and.returnValue(of(EMPTY_PAGE));
+    fixture.detectChanges();
+    expect(fixture.componentInstance.activeFilters).toEqual(['cat', 'funny']);
+    expect(listSpy).toHaveBeenCalledWith(['cat', 'funny'], jasmine.any(Number), jasmine.any(Number));
+  });
+
+  it('does not set activeFilters when no ?tags= param present', () => {
+    TestBed.overrideProvider(ActivatedRoute, { useValue: makeActivatedRoute() });
+    const fixture = TestBed.createComponent(LibraryComponent);
+    const imgSvc = TestBed.inject(ImageService);
+    spyOn(imgSvc, 'list').and.returnValue(of(EMPTY_PAGE));
+    fixture.detectChanges();
+    expect(fixture.componentInstance.activeFilters).toEqual([]);
+  });
+
+  it('header contains a link to /tags', () => {
+    const fixture = TestBed.createComponent(LibraryComponent);
+    const imgSvc = TestBed.inject(ImageService);
+    spyOn(imgSvc, 'list').and.returnValue(of(EMPTY_PAGE));
+    fixture.detectChanges();
+    const link = (fixture.nativeElement as HTMLElement).querySelector('a[href="/tags"]');
+    expect(link).not.toBeNull();
+  });
 });