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/hooks/useJSONImport.ts

@@ -0,0 +1,84 @@
+import { useState } from 'react';
+import { useMutation, useQueryClient } from '@tanstack/react-query';
+import {
+  uploadJSONExport,
+  commitJSONImport,
+  cancelJSONImport,
+  JSONImportPreview,
+  JSONImportCommitResult,
+} from '../api/jsonImport';
+
+/**
+ * Hook for managing JSON import workflow.
+ * Provides upload, commit, and cancel functionality with state management.
+ */
+export function useJSONImport() {
+  const queryClient = useQueryClient();
+  const [preview, setPreview] = useState<JSONImportPreview | null>(null);
+  const [sessionId, setSessionId] = useState<string | null>(null);
+  const [commitResult, setCommitResult] = useState<JSONImportCommitResult | null>(null);
+
+  const uploadMutation = useMutation({
+    mutationFn: uploadJSONExport,
+    onSuccess: (data) => {
+      setPreview(data);
+      setSessionId(data.session.id);
+    },
+  });
+
+  const commitMutation = useMutation({
+    mutationFn: ({
+      resolutions,
+      names,
+    }: {
+      resolutions: Record<string, string>;
+      names: Record<string, string>;
+    }) => {
+      if (!sessionId) throw new Error('No active session');
+      return commitJSONImport(sessionId, resolutions, names);
+    },
+    onSuccess: (data) => {
+      setCommitResult(data);
+      setPreview(null);
+      setSessionId(null);
+      queryClient.invalidateQueries({ queryKey: ['proxy-hosts'] });
+    },
+  });
+
+  const cancelMutation = useMutation({
+    mutationFn: cancelJSONImport,
+    onSuccess: () => {
+      setPreview(null);
+      setSessionId(null);
+    },
+  });
+
+  const clearCommitResult = () => {
+    setCommitResult(null);
+  };
+
+  const reset = () => {
+    setPreview(null);
+    setSessionId(null);
+    setCommitResult(null);
+  };
+
+  return {
+    preview,
+    sessionId,
+    loading: uploadMutation.isPending,
+    error: uploadMutation.error,
+    upload: uploadMutation.mutateAsync,
+    commit: (resolutions: Record<string, string>, names: Record<string, string>) =>
+      commitMutation.mutateAsync({ resolutions, names }),
+    committing: commitMutation.isPending,
+    commitError: commitMutation.error,
+    commitResult,
+    clearCommitResult,
+    cancel: cancelMutation.mutateAsync,
+    cancelling: cancelMutation.isPending,
+    reset,
+  };
+}
+
+export type { JSONImportPreview, JSONImportCommitResult };