fix: set default API base URL to https://api.imphnen.dev

VITE_API_URL was never set in the Nix build, so getBaseURL() returned
empty string causing all API calls to hit the current origin (e.g.
hackathon.imphnen.dev) instead of api.imphnen.dev. This caused 405
errors on login since the frontend server doesn't handle POST.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

libs/service/src/api/index.ts

@@ -67,7 +67,7 @@ export const getBaseURL = () => {
   } catch {
     // not in Vite context
   }
-  return '';
+  return 'https://api.imphnen.dev';
 };
 
 const config: AxiosRequestConfig = {