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/path-mapper.test.ts

@@ -0,0 +1,50 @@
+/**
+ * Component: Path Mapper Tests
+ * Documentation: documentation/phase3/qbittorrent.md
+ */
+
+import { describe, expect, it } from 'vitest';
+import { PathMapper } from '@/lib/utils/path-mapper';
+
+describe('PathMapper', () => {
+  it('returns original path when mapping is disabled', () => {
+    const result = PathMapper.transform('/remote/path/book', {
+      enabled: false,
+      remotePath: '/remote/path',
+      localPath: '/local/path',
+    });
+
+    expect(result).toBe('/remote/path/book');
+  });
+
+  it('transforms remote path to local path when enabled', () => {
+    const result = PathMapper.transform('/remote/mnt/d/done/Book', {
+      enabled: true,
+      remotePath: '/remote/mnt/d/done',
+      localPath: '/downloads',
+    });
+
+    expect(result.replace(/\\/g, '/')).toBe('/downloads/Book');
+  });
+
+  it('returns original path when remote prefix does not match', () => {
+    const result = PathMapper.transform('/other/path/book', {
+      enabled: true,
+      remotePath: '/remote/path',
+      localPath: '/local/path',
+    });
+
+    expect(result).toBe('/other/path/book');
+  });
+
+  it('validates mapping configuration when enabled', () => {
+    expect(() =>
+      PathMapper.validate({ enabled: true, remotePath: '', localPath: '/local' })
+    ).toThrow('Remote path cannot be empty');
+    expect(() =>
+      PathMapper.validate({ enabled: true, remotePath: '/remote', localPath: '' })
+    ).toThrow('Local path cannot be empty');
+  });
+});
+
+