Full mobile responsive (320px) and tablet (768px) support across all templates

- All text-[20rem] background text → text-[8rem] md:text-[20rem]
- All text-8xl → text-5xl md:text-8xl
- All text-5xl emojis → text-3xl md:text-5xl
- edit_event: w-full md:w-[80%], poster column w-full lg:w-[350px]
- account/index: tab bar overflow-x-auto, events table overflow-x-auto
- admin/events: table overflow-x-auto
- register: tab buttons overflow-x-auto
- error 404/500: responsive padding p-6 md:p-12
- base footer: flex-col sm:flex-row for bottom bar
- Add PWA bundle (spomky-labs/pwa-bundle) with composer require files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

assets/styles/app.css

@@ -0,0 +1,3 @@
+body {
+    background-color: skyblue;
+}