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/dutyList.js

@@ -168,10 +168,10 @@ export function renderDutyList(duties) {
         const calendarHeight = calendarSticky.offsetHeight;
         const top = el.getBoundingClientRect().top + window.scrollY;
         const scrollTop = Math.max(0, top - calendarHeight);
-        window.scrollTo({ top: scrollTop, behavior: "auto" });
+        window.scrollTo({ top: scrollTop, behavior: "smooth" });
       });
     } else {
-      el.scrollIntoView({ behavior: "auto", block: "start" });
+      el.scrollIntoView({ behavior: "smooth", block: "start" });
     }
   };
   const currentDutyCard = dutyListEl.querySelector(".duty-item--current");