fix(security): add safeStorage fallback for production builds (#38) (#43)

Summary
  - Replace hard keytar dependency with a multi-strategy key storage:
    1) safeStorage (Electron built-in) — preferred
    2) keytar (system keychain) — fallback
    3) file-based encryption — last resort
  - Dynamically import keytar to avoid asar load failures
  - Refactor cloudHandler to use const (clean code)
  - Fix “Failed to load cloud accounts” in packaged app

  Rationale
  The keytar native module can fail to load from asar archives in production builds. This ensures the app works even when system keychain access is unavailable.

  Additional changes
  - Replace sync file ops with async (fs/promises)
  - Add atomic file writes to prevent data races
  - Implement concurrency lock for key generation
  - Add security error codes for better diagnostics
  - Add warnings for file-based fallback mode
  - Align error handling after merge
  - Migrate legacy encrypted account data

  References
  - Issue: #38
  - Commit: 92dc2f6

src/tests/unit/security-migration.test.ts

@@ -0,0 +1,122 @@
+import crypto from 'crypto';
+import fs from 'fs/promises';
+import keytar from 'keytar';
+import { safeStorage } from 'electron';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { decryptWithMigration } from '../../utils/security';
+
+const primaryHex = '11'.repeat(32);
+const fallbackHex = '22'.repeat(32);
+
+vi.mock('electron', () => ({
+  safeStorage: {
+    isEncryptionAvailable: vi.fn(() => true),
+    decryptString: vi.fn(() => primaryHex),
+    encryptString: vi.fn((value: string) => Buffer.from(value)),
+  },
+  app: {
+    getPath: vi.fn(() => 'C:\\test'),
+  },
+}));
+
+vi.mock('fs/promises', () => ({
+  default: {
+    readFile: vi.fn(),
+    writeFile: vi.fn(),
+    rename: vi.fn(),
+    unlink: vi.fn(),
+  },
+}));
+
+vi.mock('keytar', () => ({
+  default: {
+    findCredentials: vi.fn(async () => []),
+    getPassword: vi.fn(async () => fallbackHex),
+    setPassword: vi.fn(),
+  },
+}));
+
+const fsMock = vi.mocked(fs, { deep: true });
+const keytarMock = vi.mocked(keytar, { deep: true });
+const safeStorageMock = vi.mocked(safeStorage, { deep: true });
+
+function encryptWithKey(key: Buffer, text: string): string {
+  const iv = crypto.randomBytes(16);
+  const cipher = crypto.createCipheriv('aes-256-gcm', key, iv);
+
+  let encrypted = cipher.update(text, 'utf8', 'hex');
+  encrypted += cipher.final('hex');
+
+  const authTag = cipher.getAuthTag();
+
+  return `${iv.toString('hex')}:${authTag.toString('hex')}:${encrypted}`;
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+
+  safeStorageMock.isEncryptionAvailable.mockReturnValue(true);
+  safeStorageMock.decryptString.mockReturnValue(primaryHex);
+
+  fsMock.readFile.mockImplementation(async (_path, encoding) => {
+    if (encoding === 'utf8') {
+      return 'not-hex';
+    }
+
+    return Buffer.from('encrypted');
+  });
+  fsMock.writeFile.mockResolvedValue(undefined);
+  fsMock.rename.mockResolvedValue(undefined);
+  fsMock.unlink.mockResolvedValue(undefined);
+
+  keytarMock.findCredentials.mockResolvedValue([]);
+  keytarMock.getPassword.mockResolvedValue(fallbackHex);
+});
+
+describe('decryptWithMigration', () => {
+  it('falls back to legacy key and re-encrypts', async () => {
+    const plaintext = '{"token":"legacy"}';
+    const ciphertext = encryptWithKey(Buffer.from(fallbackHex, 'hex'), plaintext);
+
+    const result = await decryptWithMigration(ciphertext);
+
+    expect(result.value).toBe(plaintext);
+    expect(result.usedFallback).toBe('keytar');
+    expect(result.reencrypted).toBeTypeOf('string');
+    expect(result.reencrypted).not.toBe(ciphertext);
+    expect(keytarMock.getPassword).toHaveBeenCalledTimes(1);
+
+    if (result.reencrypted) {
+      const migrated = await decryptWithMigration(result.reencrypted);
+      expect(migrated.value).toBe(plaintext);
+      expect(migrated.usedFallback).toBeUndefined();
+    }
+  });
+
+  it('does not use fallback when primary key works', async () => {
+    const plaintext = '{"token":"primary"}';
+    const ciphertext = encryptWithKey(Buffer.from(primaryHex, 'hex'), plaintext);
+
+    const result = await decryptWithMigration(ciphertext);
+
+    expect(result.value).toBe(plaintext);
+    expect(result.usedFallback).toBeUndefined();
+    expect(result.reencrypted).toBeUndefined();
+  });
+
+  it('throws migration error code when legacy keys are unavailable', async () => {
+    keytarMock.getPassword.mockResolvedValue(null);
+    fsMock.readFile.mockImplementation(async (_path, encoding) => {
+      if (encoding === 'utf8') {
+        return 'not-hex';
+      }
+
+      return Buffer.from('encrypted');
+    });
+
+    const plaintext = '{"token":"legacy"}';
+    const ciphertext = encryptWithKey(Buffer.from('33'.repeat(32), 'hex'), plaintext);
+
+    await expect(decryptWithMigration(ciphertext)).rejects.toThrow('ERR_DATA_MIGRATION_FAILED');
+  });
+});