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/LICENSE

@@ -0,0 +1,24 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
+
+  The GNU General Public License is a free, copyleft license for
+software and other kinds of works.
+
+  The licenses for most software and other practical works are designed
+to take away your freedom to share and change the works.  By contrast,
+the GNU General Public License is intended to guarantee your freedom to
+share and change all versions of a program--to make sure it remains free
+software for all its users.  We, the Free Software Foundation, use the
+GNU General Public License for most of our software; it applies also to
+any other work released this way by its authors.  You can apply it to
+your programs, too.
+
+[... Full license text continues ...]
+
+  For the full license text, visit: https://www.gnu.org/licenses/gpl-3.0.txt