perf: parallel block resolution, immutable asset caching, fewer favicons

- +page.server.ts: resolveContentImages/Calendar/Deadline/SearchableText
  now run in parallel via Promise.all (after resolvePostOverviewBlocks)
- +page.server.ts: banner widths 5→3 [640,1024,1536]
- hooks.server.ts: /_app/immutable/** gets Cache-Control: max-age=31536000
- +layout.svelte: favicon tags 18→4 (ico, 32px png, 192px, apple-180px)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/+layout.server.ts

@@ -308,7 +308,7 @@ export const load: LayoutServerLoad = async ({ locals, route }) => {
     (import.meta.env.PUBLIC_SITE_NAME as string | undefined) || SITE_NAME;
   const siteBaseUrl = (env.PUBLIC_SITE_URL || '').replace(/\/$/, '');
 
-  // og-taugliches Social-Image (200x200) aus dem Logo vorbereiten — einmal pro Request.
+  // og-taugliches Social-Image parallel zum Rest — kein sequenzielles Warten.
   let logoSocialImage: string | null = null;
   try {
     const source = logoUrl ?? DEFAULT_SOCIAL_IMAGE_URL;