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/locales/en/translation.json

@@ -69,6 +69,8 @@
     "accountManagement": "Account Management",
     "import": "Import",
     "caddyfile": "Caddyfile",
+    "importNPM": "Import NPM",
+    "importJSON": "Import JSON",
     "backups": "Backups",
     "logs": "Logs",
     "securityHeaders": "Security Headers",
@@ -761,6 +763,38 @@
     "creatingBackup": "Creating backup...",
     "importing": "Importing CrowdSec..."
   },
+  "importNPM": {
+    "title": "Import from NPM",
+    "description": "Import proxy hosts from Nginx Proxy Manager export",
+    "enterContent": "Please paste NPM export JSON",
+    "invalidJSON": "Invalid JSON format",
+    "upload": "Upload & Preview",
+    "import": "Import",
+    "success": "Import completed successfully",
+    "previewTitle": "Preview Import",
+    "conflict": "Conflict",
+    "new": "New",
+    "skip": "Skip",
+    "keep": "Keep Existing",
+    "replace": "Replace",
+    "cancelConfirm": "Are you sure you want to cancel this import?"
+  },
+  "importJSON": {
+    "title": "Import from JSON",
+    "description": "Import configuration from JSON export",
+    "enterContent": "Please paste JSON configuration",
+    "invalidJSON": "Invalid JSON format",
+    "upload": "Upload & Preview",
+    "import": "Import",
+    "success": "Import completed successfully",
+    "previewTitle": "Preview Import",
+    "conflict": "Conflict",
+    "new": "New",
+    "skip": "Skip",
+    "keep": "Keep Existing",
+    "replace": "Replace",
+    "cancelConfirm": "Are you sure you want to cancel this import?"
+  },
   "systemSettings": {
     "title": "System Settings",
     "settingsSaved": "System settings saved",