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/services/errorUtils.ts

@@ -0,0 +1,47 @@
+export const FILE_EVENTS = {
+  markError: 'files:markError',
+} as const;
+
+const UUID_REGEX = /[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}/g;
+
+export function tryParseJson<T = any>(input: unknown): T | undefined {
+  if (typeof input !== 'string') return input as T | undefined;
+  try { return JSON.parse(input) as T; } catch { return undefined; }
+}
+
+export async function normalizeAxiosErrorData(data: any): Promise<any> {
+  if (!data) return undefined;
+  if (typeof data?.text === 'function') {
+    const text = await data.text();
+    return tryParseJson(text) ?? text;
+  }
+  return data;
+}
+
+export function extractErrorFileIds(payload: any): string[] | undefined {
+  if (!payload) return undefined;
+  if (Array.isArray(payload?.errorFileIds)) return payload.errorFileIds as string[];
+  if (typeof payload === 'string') {
+    const matches = payload.match(UUID_REGEX);
+    if (matches && matches.length > 0) return Array.from(new Set(matches));
+  }
+  return undefined;
+}
+
+export function broadcastErroredFiles(fileIds: string[]) {
+  if (!fileIds || fileIds.length === 0) return;
+  window.dispatchEvent(new CustomEvent(FILE_EVENTS.markError, { detail: { fileIds } }));
+}
+
+export function isZeroByte(file: File | { size?: number } | null | undefined): boolean {
+  if (!file) return true;
+  const size = (file as any).size;
+  return typeof size === 'number' ? size <= 0 : true;
+}
+
+export function isEmptyOutput(files: File[] | null | undefined): boolean {
+  if (!files || files.length === 0) return true;
+  return files.every(f => (f as any)?.size === 0);
+}
+
+