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/scripts/patch-static-heads.mjs

@@ -19,7 +19,7 @@ const PAGES = [
   },
   {
     file: 'play.html', path: '/play',
-    title: 'Play · Upbeat Bytes — calm daily games',
+    title: 'Play · upbeatBytes — calm daily games',
     desc: 'A calm set of daily games — Daily Word, Word Search, Bloom, and Memory Match. ' +
           'A friendly little break from the doomscroll.',
   },