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/lib/components/Header.svelte

@@ -6,8 +6,8 @@
 
 <header class="appbar">
   <div class="container bar">
-    <a class="brand" href="/" aria-label="Upbeat Bytes — home">
-      <img class="logo" src="/logo.svg" alt="Upbeat Bytes" width="586" height="196" />
+    <a class="brand" href="/" aria-label="upbeatBytes — home">
+      <img class="logo" src="/logo.svg" alt="upbeatBytes" width="586" height="196" />
     </a>
 
     <nav class="utils" aria-label="Your controls">