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/torrent-categories.test.ts

@@ -0,0 +1,42 @@
+/**
+ * Component: Torrent Category Utils Tests
+ * Documentation: documentation/phase3/prowlarr.md
+ */
+
+import { describe, expect, it } from 'vitest';
+import {
+  DEFAULT_CATEGORIES,
+  TORRENT_CATEGORIES,
+  areAllChildrenSelected,
+  getChildIds,
+  getParentId,
+  isParentCategory,
+} from '@/lib/utils/torrent-categories';
+
+describe('torrent categories', () => {
+  it('returns child ids for parent categories', () => {
+    expect(getChildIds(3000)).toContain(3030);
+    expect(getChildIds(8000)).toEqual([]);
+  });
+
+  it('returns parent id for child categories', () => {
+    expect(getParentId(3030)).toBe(3000);
+    expect(getParentId(9999)).toBeNull();
+  });
+
+  it('checks if all children are selected', () => {
+    const childIds = getChildIds(3000);
+    expect(areAllChildrenSelected(3000, childIds)).toBe(true);
+    expect(areAllChildrenSelected(3000, [])).toBe(false);
+  });
+
+  it('detects parent categories', () => {
+    expect(isParentCategory(3000)).toBe(true);
+    expect(isParentCategory(3030)).toBe(false);
+  });
+
+  it('keeps default categories stable', () => {
+    expect(DEFAULT_CATEGORIES).toEqual([3030]);
+    expect(TORRENT_CATEGORIES.length).toBeGreaterThan(0);
+  });
+});