Implement M5: hardening, input validation, and Vite+React+TS dashboard

Backend hardening:
- Fix 6 nginx.go non-constant format string build errors
- Add validation.go with hostname, PEM, and enum validators
- Apply input validation to all POST/PUT handlers (certificates,
  agents, CSR, policies, teams, owners, targets, issuers)
- Fix unchecked JSON decode in TriggerDeployment handler

Frontend (Vite + React + TypeScript):
- Migrate from single-file SPA to proper build pipeline
- 7 pages: Dashboard, Certificates (list+detail), Agents, Jobs,
  Notifications, Policies, Audit Trail
- TanStack Query for server state with auto-refetch intervals
- Certificate detail with version history and renewal trigger
- Job cancellation, status/type filtering, expiry countdowns
- Reusable components: DataTable, StatusBadge, ErrorState, PageHeader
- Dark theme with Tailwind CSS, sidebar nav via React Router

Server integration:
- Go server serves web/dist/ (Vite output) with SPA fallback
- Falls back to web/index.html for legacy mode
- .gitignore updated for web/node_modules/ and web/dist/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/handler/certificate_handler_test.go

@@ -305,6 +305,9 @@ func TestCreateCertificate_Success(t *testing.T) {
 	certBody := domain.ManagedCertificate{
 		Name:       "Production Cert",
 		CommonName: "example.com",
+		OwnerID:    "o-alice",
+		TeamID:     "t-platform",
+		IssuerID:   "iss-local",
 	}
 	body, _ := json.Marshal(certBody)
 
@@ -359,6 +362,9 @@ func TestCreateCertificate_ServiceError(t *testing.T) {
 	certBody := domain.ManagedCertificate{
 		Name:       "Production Cert",
 		CommonName: "example.com",
+		OwnerID:    "o-alice",
+		TeamID:     "t-platform",
+		IssuerID:   "iss-local",
 	}
 	body, _ := json.Marshal(certBody)