brand: standardize "Upbeat Bytes" → "upbeatBytes" everywhere

Per the logo + brand: the name is upbeatBytes (camelCase). Swept all user-facing
strings — titles/og:site_name/og:title, logo alt text, share pages (share.py),
emails (email_send), classifier prompt (llm), digest/unsubscribe (api), PWA
manifest, game share text, sign-in, the SPA shell + patch-static-heads (play
title) — plus README/publish.sh and the email test fixture. (SMTP From env was
already upbeatBytes.) Domains (upbeatbytes.com) unchanged. 425 BE + 36 FE green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/zen/+page.svelte

@@ -51,13 +51,13 @@
 </script>
 
 <svelte:head>
-  <title>The Zen Den · Upbeat Bytes</title>
+  <title>The Zen Den · upbeatBytes</title>
   {#if isDevGated('zen')}<meta name="robots" content="noindex" />{/if}
 </svelte:head>
 
 <header class="bar">
   <div class="container inner">
-    <a class="brand" href="/"><img class="logo" src="/logo.svg" alt="Upbeat Bytes" /></a>
+    <a class="brand" href="/"><img class="logo" src="/logo.svg" alt="upbeatBytes" /></a>
     <a class="back" href="/play">
       <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M19 12H5M11 6l-6 6 6 6" fill="none" stroke="currentColor" stroke-width="2.1" stroke-linecap="round" stroke-linejoin="round"/></svg>Play
     </a>