Show the seating editor as a centered popup

Add Fluid.Popup, a centered dialog with the same open/close API as
Sheet: it scales in like the confirm alert, fades out in place, and
closes on backdrop tap or Escape with focus trapped inside. The host's
seating editor now uses it instead of a bottom sheet; its list scrolls
inside the popup on short screens.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/table.html

@@ -157,11 +157,10 @@
     </div>
   </div>
 
-  <!-- 席順の変更（ホストのみ）。自分から時計回りの並びを、行をつかんで並べ替える -->
-  <div class="sheet-backdrop" id="seating-modal" hidden>
-    <div class="sheet seating-sheet" role="dialog" aria-modal="true" aria-labelledby="seating-title" tabindex="-1">
-      <div class="sheet-header" data-sheet-drag>
-        <span class="sheet-grabber" aria-hidden="true"></span>
+  <!-- 席順の変更（ホストのみ・画面の中央に出るポップアップ）。自分から時計回りの並びを、行をつかんで並べ替える -->
+  <div class="popup-backdrop" id="seating-modal" hidden>
+    <div class="popup seating-popup" role="dialog" aria-modal="true" aria-labelledby="seating-title" tabindex="-1">
+      <div class="popup-header">
         <h2 class="sheet-title" id="seating-title">席順を変更</h2>
       </div>
       <div class="sheet-scroll">