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/assets/table.js

@@ -31,7 +31,7 @@
 
   const reviewSheet = Fluid.Sheet($('review-modal'));
   const rankingSheet = Fluid.Sheet($('ranking-modal'));
-  const seatingSheet = Fluid.Sheet($('seating-modal'));
+  const seatingPopup = Fluid.Popup($('seating-modal'));
   const nameSheet = Fluid.Sheet($('name-modal'), { dismissible: false, focus: '#name-input' });
   // 払って閉じた・背景を押した・Esc のときも「戻る」と同じ
   const betSheet = Fluid.Sheet($('bet-modal'), {
@@ -120,7 +120,7 @@
     $('action-bar').hidden = true;
     reviewSheet.close();
     rankingSheet.close();
-    seatingSheet.close();
+    seatingPopup.close();
     raising = false;
     betSheet.close();
     $('not-found').hidden = false;
@@ -268,7 +268,7 @@
     else betSheet.close();
     renderReview(s);
     if (rankingSheet.isOpen) renderRanking(s);
-    if (seatingSheet.isOpen) syncSeating(s);
+    if (seatingPopup.isOpen) syncSeating(s);
     renderTimer();
 
     // 自分の番になったら知らせる
@@ -774,7 +774,7 @@
   // 開いている間に座った人は最後に加え、いなくなった人は外す（並べた順は崩さない）。
   // 次のハンドで席番号が振り直されたら読み込み直す
   function syncSeating(s) {
-    if (!s.isHost) return seatingSheet.close();
+    if (!s.isHost) return seatingPopup.close();
     if (seatDrag) return;
     const occupied = occupiedSeats(s);
     const renumbered =
@@ -962,10 +962,10 @@
   $('seating-open').addEventListener('click', () => {
     if (!state || !state.isHost) return;
     loadSeating(state);
-    renderSeating(null); // 中身を入れてから開く（シートの高さを測るため）
-    seatingSheet.open();
+    renderSeating(null);
+    seatingPopup.open();
   });
-  $('seating-cancel').addEventListener('click', () => seatingSheet.close());
+  $('seating-cancel').addEventListener('click', () => seatingPopup.close());
   $('seating-save').addEventListener('click', () => {
     if (!socket || !state) return;
     const later = Boolean(state.hand); // ハンドがある間（結果の表示中を含む）は次のハンドから
@@ -978,7 +978,7 @@
         if (state) syncSeating(state);
         return;
       }
-      seatingSheet.close();
+      seatingPopup.close();
       toast(later ? '次のハンドから新しい席順になります' : '席順を変更しました');
     });
   });