feat(seo): Phase 3 — preload LCP image and reduce APP_INITIALIZER blocking

- Add <link rel="preload"> for hero background image (LCP element) in index.html
- Remove blocking APP_INITIALIZER for config.json and station config loading
- Config loads now fire asynchronously in App.ngOnInit() (browser only)
- SSR/prerender renders instantly with default config — no network wait
- Improves LCP, Time to First Byte, and initial paint for SEO

src/app/services/station-config.service.ts

@@ -6,7 +6,10 @@ import { StationConfig } from '../interfaces/station-config';
 import { getAppConfig } from './app-config.service';
 import { ThemeService } from './theme.service';
 
-/** Hardcoded defaults — used when the API is unavailable (dev fallback). */
+/**
+ * Hardcoded defaults — used when the API is unavailable (dev fallback).
+ * Rendered immediately on the initial paint so the page is never blank.
+ */
 const DEFAULT_CONFIG: StationConfig = {
   id: 0,
   key: 'default',