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/static/manifest.webmanifest

@@ -1,6 +1,6 @@
 {
-  "name": "Upbeat Bytes",
-  "short_name": "Upbeat Bytes",
+  "name": "upbeatBytes",
+  "short_name": "upbeatBytes",
   "description": "A calmer, brighter corner of the internet: good news, daily art, small games, and little resets.",
   "start_url": "/",
   "scope": "/",