Rebrand user-facing product to Upbeat Bytes (upbeatbytes.com)

Masthead, page <title>, PWA manifest name, and footer now say 'Upbeat Bytes';
README headline updated. The internal Python package/CLI stay 'goodnews' (no
functional reason to rename, and it avoids churn).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -5,7 +5,7 @@
 
 <header class="site">
   <div class="container">
-    <a class="brand" href="/">good<span>News</span></a>
+    <a class="brand" href="/">Upbeat <span>Bytes</span></a>
     <p class="tagline">Calm, constructive news worth your attention — and nothing that isn't.</p>
   </div>
 </header>
@@ -16,7 +16,7 @@
 
 <footer class="site">
   <div class="container">
-    goodNews · metadata &amp; links only, no stored articles · <a href="/docs">API</a>
+    Upbeat Bytes · metadata &amp; links only, no stored articles · <a href="/docs">API</a>
   </div>
 </footer>