refactor: organize tests and extract formatting utilities

- Move test files to lib/__tests__/ directory
- Extract formatting utilities (formatVersion, formatFileSize, formatDateTime)
  from fs-utils.ts to new utils.ts module
- Add Jest test configuration and test scripts
- Update component imports to use new utils module
- Add CLAUDE.md documentation for project structure

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/download/ApkCard.tsx

@@ -1,5 +1,5 @@
 // app/download/ApkCard.tsx
-import { formatFileSize, formatDateTime } from '@/lib/fs-utils';
+import { formatFileSize, formatDateTime } from '@/lib/utils';
 
 interface ApkCardProps {
   name: string;

app/download/SearchForm.tsx

@@ -3,7 +3,7 @@
 
 import { useState } from 'react';
 import useSWR from 'swr';
-import { formatVersion } from '@/lib/fs-utils';
+import { formatVersion } from '@/lib/utils';
 
 const fetcher = async (url: string) => {
   const res = await fetch(url);