feat(frontend): integrate error handling into application

App.svelte:
- Import and render ErrorModal component
- Add ErrorModal to main app layout below Header

CheckIfAniListLoggedInAndLoadWatchList.svelte:
- Import error state helpers (setApiError, clearApiError)
- Wrap LoadAniListUser in try-catch with error handling
- Wrap LoadAniListWatchList in try-catch with error handling
- Update CheckIfAniListLoggedInAndLoadWatchList with error handling
- Remove old alert() calls in favor of modal system

Home.svelte:
- Import isApiDown and apiError stores
- Add conditional rendering for API down state
- Display user-friendly "API Unavailable" message when apiError is set
- Show warning icon and helpful messaging

Error handling is now fully integrated across the frontend application.

frontend/src/App.svelte

@@ -25,6 +25,7 @@
     SimklGetUserWatchlist,
   } from "../wailsjs/go/main/App";
   import { loc } from "svelte-spa-router";
+  import ErrorModal from "./helperComponents/ErrorModal.svelte";
 
   onMount(async () => {
     let isAniListLoggedIn: boolean;
@@ -57,6 +58,7 @@
 </script>
 
 <Header />
+<ErrorModal />
 <Router
   routes={{
     "/": Home,