Add PWA assets, update index.astro, set .gitea

- Added PWA-related files: LICENSE, favicon.ico, icon-192.png, icon-512.png, manifest.json, service-worker.js in public/
- Deleted legacy favicon.svg
- Major update to src/pages/index.astro: implements full responsive UI, PWA manifest, service worker registration, and touch/mobile optimizations
- Added/updated .gitea to track current Gitea issue for workflow compliance

Purpose: Implements PWA support and refactors the main page for the Astro app, aligning with the migration and refactor described in issue #1.

Refs #1

public/manifest.json

@@ -0,0 +1,22 @@
+{
+  "name": "BSC Score",
+  "short_name": "BSC Score",
+  "description": "Pool Scoring App für den Billard Sport Club",
+  "lang": "de",
+  "start_url": ".",
+  "display": "fullscreen",
+  "background_color": "#1a1a1a",
+  "theme_color": "#000000",
+  "icons": [
+    {
+      "src": "icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    }
+  ]
+}