Show AdGuard filter diagnostics

[foxly-it]

What changed

• proxy the Core filter-report endpoint through the authenticated WebApp backend
• add a bilingual on-demand filter check with per-host outcome and matched rule
• distinguish expected advertising/tracking blocks from informational service and security-test checks
• show and reconcile RootGuard DNS baseline recommendations

Dependency

Requires the matching rootguard-core filter-report API change.

Validation

• backend: go test ./... and go vet ./...
• frontend: npm run lint and npm run build
• git diff --check