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.

src/app/admin/settings/tabs/PathsTab/usePathsSettings.ts

@@ -0,0 +1,84 @@
+/**
+ * Component: Paths Settings Tab - Custom Hook
+ * Documentation: documentation/settings-pages.md
+ */
+
+'use client';
+
+import { useState } from 'react';
+import type { PathsSettings, TestResult } from '../../lib/types';
+
+interface UsePathsSettingsProps {
+  paths: PathsSettings;
+  onChange: (paths: PathsSettings) => void;
+  onValidationChange: (isValid: boolean) => void;
+}
+
+export function usePathsSettings({ paths, onChange, onValidationChange }: UsePathsSettingsProps) {
+  const [testing, setTesting] = useState(false);
+  const [testResult, setTestResult] = useState<TestResult | null>(null);
+
+  /**
+   * Update a single path field
+   */
+  const updatePath = (field: keyof PathsSettings, value: string | boolean) => {
+    onChange({ ...paths, [field]: value });
+    onValidationChange(false);
+  };
+
+  /**
+   * Test if paths are valid and writable
+   */
+  const testPaths = async () => {
+    setTesting(true);
+    setTestResult(null);
+
+    try {
+      const response = await fetch('/api/setup/test-paths', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({
+          downloadDir: paths.downloadDir,
+          mediaDir: paths.mediaDir,
+        }),
+      });
+
+      const data = await response.json();
+
+      if (data.success) {
+        const result: TestResult = {
+          success: true,
+          message: 'All paths are valid and writable'
+        };
+        setTestResult(result);
+        onValidationChange(true);
+        return result;
+      } else {
+        const result: TestResult = {
+          success: false,
+          message: data.error || 'Path validation failed'
+        };
+        setTestResult(result);
+        onValidationChange(false);
+        return result;
+      }
+    } catch (error) {
+      const result: TestResult = {
+        success: false,
+        message: error instanceof Error ? error.message : 'Failed to test paths'
+      };
+      setTestResult(result);
+      onValidationChange(false);
+      return result;
+    } finally {
+      setTesting(false);
+    }
+  };
+
+  return {
+    testing,
+    testResult,
+    updatePath,
+    testPaths,
+  };
+}