Add backend unit test framework and modularize settings UI

Introduced a Vitest-based backend unit testing framework with supporting scripts, helpers, and GitHub Actions integration. Refactored the admin settings page to a modular architecture, splitting monolithic logic into feature-specific tabs and hooks for improved maintainability and testability. Updated documentation to reflect the new testing setup and settings architecture, and added new dependencies for testing utilities.

tests/services/encryption.service.test.ts

@@ -0,0 +1,54 @@
+/**
+ * Component: Encryption Service Tests
+ * Documentation: documentation/backend/services/config.md
+ */
+
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+const ORIGINAL_KEY = process.env.CONFIG_ENCRYPTION_KEY;
+
+afterEach(() => {
+  process.env.CONFIG_ENCRYPTION_KEY = ORIGINAL_KEY;
+  vi.resetModules();
+});
+
+describe('EncryptionService', () => {
+  it('throws when encryption key is missing', async () => {
+    delete process.env.CONFIG_ENCRYPTION_KEY;
+    vi.resetModules();
+
+    const { EncryptionService } = await import('@/lib/services/encryption.service');
+    expect(() => new EncryptionService()).toThrow(/CONFIG_ENCRYPTION_KEY/);
+  });
+
+  it('encrypts and decrypts values', async () => {
+    process.env.CONFIG_ENCRYPTION_KEY = 'a'.repeat(32);
+    vi.resetModules();
+
+    const { EncryptionService } = await import('@/lib/services/encryption.service');
+    const service = new EncryptionService();
+
+    const encrypted = service.encrypt('secret');
+    const decrypted = service.decrypt(encrypted);
+
+    expect(decrypted).toBe('secret');
+  });
+
+  it('rejects invalid encrypted data formats', async () => {
+    process.env.CONFIG_ENCRYPTION_KEY = 'b'.repeat(32);
+    vi.resetModules();
+
+    const { EncryptionService } = await import('@/lib/services/encryption.service');
+    const service = new EncryptionService();
+
+    expect(() => service.decrypt('invalid')).toThrow(/Decryption failed/);
+  });
+
+  it('generates a random key', async () => {
+    const { EncryptionService } = await import('@/lib/services/encryption.service');
+    const key = EncryptionService.generateKey();
+
+    expect(typeof key).toBe('string');
+    expect(key.length).toBeGreaterThan(40);
+  });
+});