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/utils/url.test.ts

@@ -0,0 +1,67 @@
+/**
+ * Component: URL Utilities Tests
+ * Documentation: documentation/backend/services/environment.md
+ */
+
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { getBaseUrl, getCallbackUrl } from '@/lib/utils/url';
+
+const envBackup = { ...process.env };
+
+describe('URL utilities', () => {
+  beforeEach(() => {
+    process.env = { ...envBackup };
+  });
+
+  afterEach(() => {
+    process.env = { ...envBackup };
+  });
+
+  it('prefers PUBLIC_URL and trims trailing slashes', () => {
+    process.env.PUBLIC_URL = 'https://example.com/';
+    process.env.NEXTAUTH_URL = 'https://next.example.com';
+    process.env.BASE_URL = 'https://base.example.com';
+
+    const url = getBaseUrl();
+
+    expect(url).toBe('https://example.com');
+  });
+
+  it('falls back to NEXTAUTH_URL when PUBLIC_URL is not set', () => {
+    delete process.env.PUBLIC_URL;
+    process.env.NEXTAUTH_URL = 'https://next.example.com/';
+
+    const url = getBaseUrl();
+
+    expect(url).toBe('https://next.example.com');
+  });
+
+  it('uses BASE_URL and keeps invalid scheme values', () => {
+    delete process.env.PUBLIC_URL;
+    delete process.env.NEXTAUTH_URL;
+    process.env.BASE_URL = 'example.com/';
+
+    const url = getBaseUrl();
+
+    expect(url).toBe('example.com');
+  });
+
+  it('defaults to localhost in production when no env vars are set', () => {
+    delete process.env.PUBLIC_URL;
+    delete process.env.NEXTAUTH_URL;
+    delete process.env.BASE_URL;
+    process.env.NODE_ENV = 'production';
+
+    const url = getBaseUrl();
+
+    expect(url).toBe('http://localhost:3030');
+  });
+
+  it('builds callback URLs with normalized paths', () => {
+    process.env.PUBLIC_URL = 'https://example.com';
+
+    const url = getCallbackUrl('api/auth/oidc/callback');
+
+    expect(url).toBe('https://example.com/api/auth/oidc/callback');
+  });
+});