chore: add ESLint, Prettier and Jest configuration

.eslintignore

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+node_modules/
+frontend/node_modules/
+frontend/.next/
+logs/
+*.log
+accountbook.db
+.worktrees/

.eslintrc.js

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+module.exports = {
+  root: true,
+  env: {
+    node: true,
+    es2021: true,
+    jest: true,
+  },
+  extends: ['eslint:recommended'],
+  parserOptions: {
+    ecmaVersion: 'latest',
+    sourceType: 'module',
+  },
+  rules: {
+    'no-console': 'off',
+    'no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
+    'prefer-const': 'error',
+    'no-var': 'error',
+  },
+};

.prettierrc

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+{
+  "semi": true,
+  "singleQuote": true,
+  "tabWidth": 2,
+  "trailingComma": "es5",
+  "printWidth": 100,
+  "bracketSpacing": true,
+  "arrowParens": "always"
+}

jest.config.js

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+module.exports = {
+  testEnvironment: 'node',
+  roots: ['<rootDir>/server'],
+  testMatch: ['**/__tests__/**/*.test.js'],
+  verbose: true,
+};

server/__tests__/validators.test.js

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+const {
+  isNonEmptyString,
+  isValidMonth,
+  isValidDateTime,
+  validateRecordPayload,
+} = require('../utils/validators');
+
+describe('Validators', () => {
+  describe('isNonEmptyString', () => {
+    test('returns true for non-empty string', () => {
+      expect(isNonEmptyString('hello')).toBe(true);
+    });
+
+    test('returns false for empty string', () => {
+      expect(isNonEmptyString('')).toBe(false);
+    });
+
+    test('returns false for non-string', () => {
+      expect(isNonEmptyString(123)).toBe(false);
+      expect(isNonEmptyString(null)).toBe(false);
+    });
+  });
+
+  describe('isValidMonth', () => {
+    test('returns true for valid month format', () => {
+      expect(isValidMonth('2024-01')).toBe(true);
+    });
+
+    test('returns false for invalid month format', () => {
+      expect(isValidMonth('2024-1')).toBe(false);
+    });
+  });
+
+  describe('validateRecordPayload', () => {
+    test('returns null for valid payload', () => {
+      const payload = {
+        type: 'expense',
+        amount: 100,
+        category: '餐饮',
+        date: '2024-01-15',
+      };
+      expect(validateRecordPayload(payload)).toBeNull();
+    });
+
+    test('returns error for invalid type', () => {
+      const payload = {
+        type: 'invalid',
+        amount: 100,
+        category: '餐饮',
+        date: '2024-01-15',
+      };
+      expect(validateRecordPayload(payload)).toBe('账目类型错误');
+    });
+  });
+});