Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/utils/fileIdSafety.ts

@@ -0,0 +1,14 @@
+/**
+ * Runtime validation utilities for FileId safety
+ */
+
+import { FileId } from '@app/types/fileContext';
+
+// Validate that a string is a proper FileId (has UUID format)
+export function isValidFileId(id: string): id is FileId {
+  // Check UUID v4 format: 8-4-4-4-12 hex digits
+  const uuidRegex = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+  return uuidRegex.test(id);
+}
+
+