feat: fix test collection and add collection stats display

- Fix collect_tests to use JSON report parsing (was returning 0 tests)
- Add Test Collection panel to testing dashboard showing total tests,
  unit/integration/performance breakdown, and file count
- Reorganize sidebar: create Platform Health section with Testing Hub,
  Code Quality, and Background Tasks
- Keep Developer Tools for Components and Icons only
- Platform Monitoring now contains Import Jobs and Application Logs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/tests.py

@@ -85,6 +85,12 @@ class TestDashboardStatsResponse(BaseModel):
 
     # Collection stats
     total_test_files: int
+    collected_tests: int
+    unit_tests: int
+    integration_tests: int
+    performance_tests: int
+    system_tests: int
+    last_collected: str | None
 
     # Trend and breakdown data
     trend: list[dict]