feat: add SSR and prerendering support

- Install @angular/ssr and @angular/platform-server (v21.2.x)
- Create server entry point (server.ts) and server bootstrap (main.server.ts)
- Add server routes config (app.server.routes.ts) with 5 prerendered routes:
  /, /about, /schedule, /events, /contact
- Update angular.json with server/ssr/prerender build configuration
- Add prerender-routes.txt for build-time static route generation
- Fix SSR compatibility:
  - App component: guard DOM access with isPlatformBrowser
  - StructuredDataComponent: skip head manipulation on server
  - ThemeService: skip CSS variable application on server
  - ShowService/EventService: return empty arrays on API failure
    (graceful fallback during prerender with no backend)
- Prerendered output verified: 5 static HTML routes in dist/

src/app/app.ts

@@ -1,4 +1,5 @@
-import { Component, computed, effect, inject, OnInit, OnDestroy } from '@angular/core';
+import { Component, computed, effect, inject, OnInit, OnDestroy, PLATFORM_ID } from '@angular/core';
+import { isPlatformBrowser } from '@angular/common';
 import { RouterOutlet } from '@angular/router';
 import { NavbarComponent } from './navbar/navbar.component';
 import { MediaPlayerComponent } from './media-player/media-player.component';
@@ -17,6 +18,7 @@ import { SeoService } from './services/seo.service';
 export class App implements OnInit, OnDestroy {
   private stationConfig = inject(StationConfigService);
   private seoService = inject(SeoService);
+  private platformId = inject(PLATFORM_ID);
 
   readonly hasStream = computed(() => {
     const cfg = this.stationConfig.config();
@@ -27,6 +29,9 @@ export class App implements OnInit, OnDestroy {
   private resizeObserver: ResizeObserver | null = null;
 
   constructor() {
+    // These DOM operations only run in the browser.
+    if (!isPlatformBrowser(this.platformId)) return;
+
     // Update favicon when station config changes.
     effect(() => {
       const config = this.stationConfig.config();
@@ -46,6 +51,9 @@ export class App implements OnInit, OnDestroy {
   }
 
   ngOnInit(): void {
+    // These DOM operations only run in the browser.
+    if (!isPlatformBrowser(this.platformId)) return;
+
     // Start watching router for automatic SEO metadata updates per route.
     this.seoService.watchRouter();
     // Handle the initial route (NavigationEnd already fired before ngOnInit).
@@ -64,7 +72,9 @@ export class App implements OnInit, OnDestroy {
   }
 
   ngOnDestroy(): void {
-    this.resizeObserver?.disconnect();
+    if (isPlatformBrowser(this.platformId)) {
+      this.resizeObserver?.disconnect();
+    }
   }
 
   private updateNavbarHeight(): void {