fix: mobile dialog and overlay panel positioning

Center PWA dialogs again by dropping the stale iOS offset that pushed a
flex-centered (position: relative) dialog down by half the viewport.

Seat the mobile overlay panel flush to the bottom (drop the float margin),
drop its bottom border in favor of a scroll shadow on the content, and raise
it above sheets (z-60) so an overlay opened over a surface sheet renders on
top instead of behind it.

packages/ui/src/styles/mobile.css

@@ -419,17 +419,6 @@
     top: calc(0.2rem + (var(--oc-safe-area-top, 0px) * 0.3));
   }
 
-  /* iOS-only: the original PWA dialog was offset down to clear the
-     status bar / notch. Android centers fine on its own and the extra
-     `top` + translate combo pushed dialogs (and their action buttons)
-     below the visible viewport, so keep this scoped to iOS. */
-  @supports (-webkit-touch-callout: none) {
-    .pwa-dialog-content {
-      top: calc(50% + (var(--oc-safe-area-top, 0px) * 0.22));
-      --tw-translate-y: calc(-50% + (var(--oc-safe-area-top, 0px) * 0.6));
-    }
-  }
-
   .pwa-dialog-content .settings-page-body {
     max-width: 100%;
     padding: 0.75rem 1rem;
@@ -449,8 +438,7 @@
   }
 
   .pwa-overlay-panel {
-    margin-bottom: max(var(--oc-safe-area-bottom-visual, 0px), 12px);
-    padding-bottom: calc(var(--oc-safe-area-bottom-visual, 0px) * 0.4);
+    margin-bottom: 0;
   }
 
   .pwa-overlay-panel .pwa-overlay-scroll {