refactor: migrate UI to Preact components and remove legacy Astro/JS

- Replaced all .astro components with .jsx Preact components and added corresponding CSS modules.
- Updated index.astro to use the new App Preact component; removed legacy script and Astro imports.
- Deleted obsolete .astro component files and main JS logic (src/scripts/index.js, public/scripts/index.js).
- Updated astro.config.mjs for Preact integration.
- Updated package.json and package-lock.json to include @astrojs/preact and preact.
- Updated tsconfig.json for Preact JSX support.
- Refactored index.css to keep only global resets and utility styles.
- All changes relate to Gitea issue #1 (refactor to astro app).

Migrates the UI from Astro/vanilla JS to a modular Preact component architecture, removing all legacy code and aligning the project with modern best practices.
Refs #1

src/components/FullscreenToggle.module.css

@@ -0,0 +1,37 @@
+.fullscreenToggle {
+    position: fixed;
+    bottom: 20px;
+    right: 20px;
+    width: 48px;
+    height: 48px;
+    border-radius: 50%;
+    background-color: rgba(52, 152, 219, 0.9);
+    border: none;
+    color: white;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    box-shadow: 0 2px 5px rgba(0, 0, 0, 0.2);
+    z-index: 9999;
+    transition: background-color 0.2s, transform 0.2s;
+}
+.fullscreenToggle:hover {
+    background-color: rgba(52, 152, 219, 1);
+    transform: scale(1.1);
+}
+.fullscreenToggle:active {
+    transform: scale(0.95);
+}
+.fullscreenToggle svg {
+    width: 24px;
+    height: 24px;
+}
+@media screen and (max-width: 480px) {
+    .fullscreenToggle {
+        bottom: 15px;
+        right: 15px;
+        width: 40px;
+        height: 40px;
+    }
+}