chore: implement NPM/JSON import routes and fix SMTP persistence

Phase 3 of skipped tests remediation - enables 7 previously skipped E2E tests

Backend:

Add NPM import handler with session-based upload/commit/cancel
Add JSON import handler with Charon/NPM format support
Fix SMTP SaveSMTPConfig using transaction-based upsert
Add comprehensive unit tests for new handlers
Frontend:

Add ImportNPM page component following ImportCaddy pattern
Add ImportJSON page component with format detection
Add useNPMImport and useJSONImport React Query hooks
Add API clients for npm/json import endpoints
Register routes in App.tsx and navigation in Layout.tsx
Add i18n keys for new import pages
Tests:

7 E2E tests now enabled and passing
Backend coverage: 86.8%
Reduced total skipped tests from 98 to 91
Closes: Phase 3 of skipped-tests-remediation plan

frontend/src/App.tsx

@@ -16,6 +16,8 @@ const RemoteServers = lazy(() => import('./pages/RemoteServers'))
 const DNS = lazy(() => import('./pages/DNS'))
 const ImportCaddy = lazy(() => import('./pages/ImportCaddy'))
 const ImportCrowdSec = lazy(() => import('./pages/ImportCrowdSec'))
+const ImportNPM = lazy(() => import('./pages/ImportNPM'))
+const ImportJSON = lazy(() => import('./pages/ImportJSON'))
 const Certificates = lazy(() => import('./pages/Certificates'))
 const DNSProviders = lazy(() => import('./pages/DNSProviders'))
 const SystemSettings = lazy(() => import('./pages/SystemSettings'))
@@ -109,6 +111,8 @@ export default function App() {
                 <Route path="import">
                   <Route path="caddyfile" element={<ImportCaddy />} />
                   <Route path="crowdsec" element={<ImportCrowdSec />} />
+                  <Route path="npm" element={<ImportNPM />} />
+                  <Route path="json" element={<ImportJSON />} />
                 </Route>
               </Route>