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/app-config.service.ts

@@ -1,5 +1,5 @@
 import { HttpClient } from '@angular/common/http';
-import { APP_INITIALIZER, InjectionToken } from '@angular/core';
+import { InjectionToken } from '@angular/core';
 import { lastValueFrom } from 'rxjs';
 
 import { environment } from '../../environments/environment';
@@ -16,15 +16,19 @@ const _appConfig: AppConfig = {
   apiBaseUrl: environment.apiBaseUrl,
 };
 
-export function appConfigFactory(http: HttpClient) {
-  return (): Promise<void> => {
-    return lastValueFrom(http.get<AppConfig>('config.json')).then(
-      (config) => { Object.assign(_appConfig, config); },
-      () => {
-        // Keep defaults if config.json is not available (dev mode).
-      },
-    );
-  };
+/**
+ * Load runtime config.json asynchronously AFTER the app boots.
+ * Replaces the former APP_INITIALIZER that blocked the initial render.
+ * This way the page renders immediately with environment defaults, then
+ * patches in the runtime API URL once it arrives.
+ */
+export function loadAppConfigAsync(http: HttpClient): Promise<void> {
+  return lastValueFrom(http.get<AppConfig>('config.json')).then(
+    (config) => { Object.assign(_appConfig, config); },
+    () => {
+      // Keep defaults if config.json is not available (dev mode).
+    },
+  );
 }
 
 export const appConfigProvider = {
@@ -32,12 +36,12 @@ export const appConfigProvider = {
   useValue: _appConfig,
 };
 
-export const appConfigInitProvider = {
-  provide: APP_INITIALIZER,
-  useFactory: appConfigFactory,
-  deps: [HttpClient],
-  multi: true,
-};
+/**
+ * @deprecated The APP_INITIALIZER is no longer registered by default.
+ * The `loadAppConfigAsync` function is called directly from `app.config.ts`
+ * in a non-blocking way. This export is kept for backward compatibility only.
+ */
+export const appConfigInitProvider = null;
 
 export function getAppConfig(): AppConfig {
   return _appConfig;

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',