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/index.html

@@ -21,7 +21,7 @@
       <div class="calendar" id="calendar"></div>
     </div>
     <div class="duty-list" id="dutyList"></div>
-    <div class="loading" id="loading">Загрузка…</div>
+    <div class="loading" id="loading"><span class="loading__spinner" aria-hidden="true"></span><span class="loading__text">Загрузка…</span></div>
     <div class="error" id="error" hidden></div>
     <div class="access-denied" id="accessDenied" hidden>
       <p>Доступ запрещён.</p>