feat: enhance loading indicator and improve UI transitions

- Updated the loading indicator in `index.html` to include a spinner and text for better user feedback during loading.
- Added CSS transitions for smoother visual effects on various elements, including buttons and overlays, enhancing overall user experience.
- Refactored JavaScript functions to utilize requestAnimationFrame for improved animation performance when showing and hiding panels.
- Implemented smooth scrolling behavior for duty list navigation, improving usability when focusing on current duties.

webapp/js/main.js

@@ -38,7 +38,8 @@ initTheme();
 state.lang = getLang();
 document.documentElement.lang = state.lang;
 document.title = t(state.lang, "app.title");
-if (loadingEl) loadingEl.textContent = t(state.lang, "loading");
+const loadingTextEl = loadingEl ? loadingEl.querySelector(".loading__text") : null;
+if (loadingTextEl) loadingTextEl.textContent = t(state.lang, "loading");
 const dayLabels = weekdayLabels(state.lang);
 if (weekdaysEl) {
   const spans = weekdaysEl.querySelectorAll("span");