Add metadata Pinia store for ExifTool queue preview reads.

Introduce preview entry types, resolveErrorMessage, and nullcart-style resolveAxiosErrorMessage for upcoming upload API errors.

frontend/src/utils/http/resolveAxiosErrorMessage.ts

@@ -1,23 +1,40 @@
-import axios from 'axios';
+import { isAxiosError } from 'axios';
+import { resolveErrorMessage } from '@/utils/resolveErrorMessage';
 
-export function resolveAxiosErrorMessage(error: unknown, fallback = 'Request failed'): string {
-    if (axios.isAxiosError(error)) {
-        const data = error.response?.data;
-        if (typeof data === 'object' && data !== null && 'message' in data) {
-            const message = (data as { message: unknown }).message;
-            if (typeof message === 'string') {
-                return message;
-            }
-            if (Array.isArray(message)) {
-                return message.map(String).join(', ');
-            }
+const getAxiosErrorResponseMessage = (data: unknown): string | null => {
+    if (typeof data === 'string') {
+        return data;
+    }
+
+    if (typeof data !== 'object' || data === null || !('message' in data)) {
+        return null;
+    }
+
+    const { message } = data;
+
+    if (typeof message === 'string') {
+        return message;
+    }
+
+    if (Array.isArray(message)) {
+        const texts = message.filter((item): item is string => typeof item === 'string');
+
+        if (texts.length === 1) {
+            return texts[0];
         }
-        if (error.message) {
-            return error.message;
+
+        if (texts.length > 1) {
+            return texts.join(', ');
         }
     }
-    if (error instanceof Error) {
-        return error.message;
+
+    return null;
+};
+
+export const resolveAxiosErrorMessage = (error: unknown, fallback: string): string => {
+    if (!isAxiosError(error) || !error.response) {
+        return resolveErrorMessage(error, fallback);
     }
-    return fallback;
-}
+
+    return getAxiosErrorResponseMessage(error.response.data) ?? fallback;
+};