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/contracts/tags-endpoint.md

@@ -0,0 +1,58 @@
+# Contract: GET /api/v1/tags (enhanced)
+
+## Overview
+
+Extends the existing tags list endpoint with two new optional query parameters. All existing behaviour is preserved when the new parameters are omitted.
+
+## Request
+
+```
+GET /api/v1/tags
+```
+
+### Query Parameters
+
+| Parameter  | Type    | Default  | Description |
+|------------|---------|----------|-------------|
+| `q`        | string  | —        | Filter tags by name prefix (existing) |
+| `limit`    | integer | 100      | Max items to return; capped at 200 (existing) |
+| `offset`   | integer | 0        | Pagination offset (existing) |
+| `sort`     | string  | `name`   | Sort order: `name` (alphabetical asc) or `count_desc` (image count descending, alphabetical secondary) |
+| `min_count`| integer | 0        | Exclude tags with fewer than this many images. Use `1` to hide zero-count tags. |
+
+### Authentication
+
+Not required. Public endpoint.
+
+## Response
+
+```json
+{
+  "items": [
+    { "id": "uuid", "name": "string", "image_count": 0 }
+  ],
+  "total": 0,
+  "limit": 100,
+  "offset": 0
+}
+```
+
+No changes to the response shape.
+
+## Tag Browser Usage
+
+The tag browser component calls:
+
+```
+GET /api/v1/tags?sort=count_desc&min_count=1&limit=500
+```
+
+`limit=500` is a safe upper bound for a personal library. If `total` exceeds `limit` in the response, the component logs a warning but renders what it received (no pagination UI required at this scale).
+
+## Library Autocomplete Usage (unchanged)
+
+```
+GET /api/v1/tags?q=<prefix>&limit=10
+```
+
+Uses neither `sort` nor `min_count` — default behaviour is unchanged.