Fix: Resolve Plex authentication cookie and polling issues

- Export CLIENT_IDENTIFIER and APP_NAME as module-level constants
  - Ensures same identifier used across all composables and API calls
  - Prevents auth failures from mismatched client identifiers

- Refactor PlexSettings.vue to use composable auth flow
  - Remove duplicate authentication logic (138 lines removed)
  - Use openAuthPopup() from usePlexAuth composable
  - Use cleanup() function in onUnmounted hook
  - Reduced from 498 lines to 360 lines (28% further reduction)

- Fix usePlexAuth to import constants directly
  - Previously tried to get constants from usePlexApi() instance
  - Now imports as shared module exports
  - Ensures consistent CLIENT_IDENTIFIER across auth flow

Total PlexSettings.vue reduction: 2094 → 360 lines (83% reduction)
Authentication flow now properly sets cookies and completes polling ✓

src/composables/usePlexAuth.ts

@@ -1,9 +1,7 @@
 import { ref } from "vue";
-import { usePlexApi } from "./usePlexApi";
+import { CLIENT_IDENTIFIER, APP_NAME } from "./usePlexApi";
 
 export function usePlexAuth() {
-  const { CLIENT_IDENTIFIER, APP_NAME } = usePlexApi();
-
   const loading = ref(false);
   const plexPopup = ref<Window | null>(null);
   const pollInterval = ref<number | null>(null);