feat(frontend): add API error state management

Add centralized error state system:
- New ApiError interface (service, message, statusCode, canRetry)
- apiError writable store for current error state
- isApiDown writable store for API availability status
- setApiError() helper to set error states
- clearApiError() helper to reset error states

Provides reactive error state across entire application.

frontend/src/helperModules/GlobalVariablesAndHelperFunctions.svelte

@@ -77,6 +77,33 @@
   aniListAnime.subscribe((value) => (currentAniListAnime = value));
   aniListSort.subscribe((value) => (sort = value));
 
+  export interface ApiError {
+    service: string;
+    message: string;
+    statusCode?: string;
+    canRetry: boolean;
+  }
+  export const apiError = writable<ApiError | null>(null);
+  export const isApiDown = writable(false);
+  export function setApiError(
+    service: string,
+    message: string,
+    statusCode?: string,
+    canRetry: boolean = true,
+  ) {
+    apiError.set({
+      service,
+      message,
+      statusCode,
+      canRetry,
+    });
+    isApiDown.set(true);
+  }
+  export function clearApiError() {
+    apiError.set(null);
+    isApiDown.set(false);
+  }
+
   export async function GetAnimeSingleItem(
     aniId: number,
     login: boolean,