feat: add API key encryption utilities using Electron safeStorage

Implement encryption utilities for securely storing API keys:
- encryptApiKey(): encrypts keys using Electron safeStorage with base64 encoding
- decryptApiKey(): decrypts stored keys
- isEncryptionAvailable(): checks platform support

Encryption falls back to plaintext on platforms without keyring support.
Includes comprehensive test coverage with Electron mock.

tests/__mocks__/electron.ts

@@ -0,0 +1,13 @@
+/**
+ * Mock Electron API for testing
+ * This provides minimal mocks for the Electron types used in tests
+ */
+
+export const safeStorage = {
+	isEncryptionAvailable: jest.fn(() => true),
+	encryptString: jest.fn((data: string) => Buffer.from(`encrypted:${data}`)),
+	decryptString: jest.fn((buffer: Buffer) => {
+		const str = buffer.toString();
+		return str.replace('encrypted:', '');
+	})
+};