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

tests/settings/smtp-settings.spec.ts

@@ -331,9 +331,9 @@ test.describe('SMTP Settings', () => {
 
     /**
      * Test: Update existing SMTP configuration
-     * Note: Skip - SMTP save not persisting correctly (backend issue, not test issue)
+     * Priority: P0
      */
-    test.skip('should update existing SMTP configuration', async ({ page }) => {
+    test('should update existing SMTP configuration', async ({ page }) => {
       const hostInput = page.locator('#smtp-host');
       const saveButton = page.getByRole('button', { name: /save/i }).last();