Fix @ alias not resolving inside wxt.config.ts

The @/ path alias is only wired up for the app bundle via WXT's
generated tsconfig; wxt.config.ts itself runs outside that resolution,
so the aliased import of BACKEND_HTTP_URL failed there. Use a relative
import instead.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Tn7uDYjTuZEbLPwpEiSCUw

wxt.config.ts

@@ -1,4 +1,4 @@
-import { BACKEND_HTTP_URL } from '@/shared/config';
+import { BACKEND_HTTP_URL } from './src/shared/config';
 import { defineConfig } from 'wxt';
 
 // See https://wxt.dev/api/config.html