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>

static/admin/js/testing-dashboard.js

@@ -38,6 +38,12 @@ function testingDashboard() {
             last_run: null,
             last_run_status: null,
             total_test_files: 0,
+            collected_tests: 0,
+            unit_tests: 0,
+            integration_tests: 0,
+            performance_tests: 0,
+            system_tests: 0,
+            last_collected: null,
             trend: [],
             by_category: {},
             top_failing: []