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/static/manifest.webmanifest

@@ -1,6 +1,6 @@
 {
-  "name": "goodNews",
-  "short_name": "goodNews",
+  "name": "Upbeat Bytes",
+  "short_name": "Upbeat Bytes",
   "description": "Calm, constructive news worth your attention — and nothing that isn't.",
   "start_url": "/",
   "scope": "/",