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>

deploy/publish.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# Publish Upbeat Bytes: build the frontend, sync it to the live Caddy site,
+# Publish upbeatBytes: build the frontend, sync it to the live Caddy site,
 # rebuild/restart the API container, and reload Caddy. One command to redeploy.
 set -euo pipefail
 
@@ -26,4 +26,4 @@ curl -fsS -o /dev/null -w '  logo-email.png → %{http_code} %{content_type}\n'
   https://upbeatbytes.com/logo-email.png \
   || echo "  ⚠ logo-email.png is not being served — the digest masthead would break!"
 
-echo "✓ Published Upbeat Bytes → https://upbeatbytes.com"
+echo "✓ Published upbeatBytes → https://upbeatbytes.com"