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/style.css

@@ -1026,6 +1026,44 @@ a {
   letter-spacing: -0.015em;
 }
 
+/* ---------- ポップアップ（fluid.js の Popup）: 画面の中央に出る大きめのダイアログ ---------- */
+.popup-backdrop {
+  --appear: 1;
+  position: fixed;
+  inset: 0;
+  z-index: 30;
+  display: grid;
+  place-items: center;
+  padding: max(16px, env(safe-area-inset-top)) 16px max(16px, env(safe-area-inset-bottom));
+}
+/* 暗さは現れ方に合わせて変わる */
+.popup-backdrop::before {
+  content: "";
+  position: absolute;
+  inset: 0;
+  background: var(--scrim);
+  opacity: var(--appear);
+}
+.popup {
+  --bg-2: var(--sheet-cell);
+  position: relative;
+  width: min(440px, 100%);
+  max-height: 100%;
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+  border-radius: 22px;
+  background: var(--sheet-bg);
+  box-shadow: var(--shadow-l), inset 0 0 0 0.5px var(--material-edge);
+  outline: none;
+  will-change: transform, opacity;
+}
+.popup-header {
+  flex: none;
+  padding: 16px 20px 6px;
+  text-align: center;
+}
+
 /* ---------- アラート（App.confirm）: 少し大きい所から現れ、その場で消える ---------- */
 .alert-backdrop {
   --appear: 1;