chore: increase LARGE_FILE_THRESHOLD from 500KB to 5MB

Raise the threshold for triggering large file handling from 500KB to 5MB.
This allows more files to benefit from full content persistence in
LocalStorage while still protecting against quota issues with very large files.

Author: claude

src/App.jsx

@@ -12,7 +12,7 @@ import { PanelLeftClose, PanelLeftOpen } from 'lucide-react';
 import { mergeFilesWithReplacement } from './utils/mergeFiles.js';
 
 // Threshold for "large file" - files above this won't have content persisted
-const LARGE_FILE_THRESHOLD = 500 * 1024; // 500KB of content
+const LARGE_FILE_THRESHOLD = 5 * 1024 * 1024; // 5MB of content
 
 // Default global parsing configuration
 export const DEFAULT_GLOBAL_PARSING_CONFIG = {