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/hooks.server.ts

@@ -115,7 +115,10 @@ export const handle: Handle = async ({ event, resolve }) => {
     event.url.searchParams.has('preview_draft');
   const isPreviewDraft = event.url.searchParams.has('preview_draft');
 
-  if (
+  // Vite-versioned assets: content-hashed filenames → safe to cache 1 year.
+  if (event.url.pathname.startsWith('/_app/immutable/')) {
+    response.headers.set('cache-control', 'public, max-age=31536000, immutable');
+  } else if (
     event.request.method === 'GET' &&
     isCacheablePath(event.url.pathname) &&
     !isPreview &&