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>

specs/007-tag-browser/quickstart.md

@@ -0,0 +1,45 @@
+# Quickstart: Tag Browser
+
+## Verifying the feature end-to-end
+
+### Prerequisites
+
+- Docker stack running (`docker compose up`)
+- At least 3 images uploaded with different tags (e.g., `cat`, `funny`, `reaction`)
+- At least one image with two tags (e.g., both `cat` and `funny`)
+
+### Scenario 1 — Tag browser shows all tags with correct counts
+
+1. Open the app (not logged in).
+2. Navigate to `/tags`.
+3. **Expected**: A list of tags is shown. Each tag displays the number of images with that tag. Tags are ordered from most images to fewest.
+4. Verify: Count next to `cat` matches the number of images actually tagged `cat`.
+5. Verify: Tags with zero images are not shown.
+
+### Scenario 2 — Clicking a tag navigates to the filtered library
+
+1. On the `/tags` page, click the `cat` tag.
+2. **Expected**: Navigated to the library (`/`) showing only images tagged `cat`.
+3. Verify: The active filter chip shows `cat` in the library.
+
+### Scenario 3 — Library page links to tag browser
+
+1. Navigate to `/` (library, logged in or out).
+2. **Expected**: A link or button labelled "Browse by tag" (or similar) is visible.
+3. Click it.
+4. **Expected**: The tag browser page loads.
+
+### Scenario 4 — Empty state
+
+1. If the library has no images at all, navigate to `/tags`.
+2. **Expected**: An empty state message is shown rather than a blank page or error.
+
+### API verification
+
+```bash
+# Sorted by count, zero-count tags excluded
+curl http://localhost:8000/api/v1/tags?sort=count_desc&min_count=1
+
+# Existing autocomplete behaviour unchanged
+curl http://localhost:8000/api/v1/tags?q=ca&limit=10
+```