fix(web): honor prefers-reduced-motion; contain overscroll in modal surfaces (#71)

Nothing in the app respected prefers-reduced-motion — the kit's
data-open/closed animations, the skeleton pulse, and the sidebar width
transition all ran unconditionally. Add a global base-layer rule that
collapses animation/transition durations to a single frame when the OS
asks for reduced motion; one rule covers current and future additions.

Add overscroll-y-contain to the scrollable modal/popup surfaces
(DrawerContent, SelectContent, ComboboxPopup) so flicking past the end
of their content no longer chain-scrolls the page beneath, and to
AlertDialogContent for when it gains scrollable content.

Verified in the built CSS: the media query and
overscroll-behavior-y:contain both compile into dist/assets.

Closes #71

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/index.css

@@ -99,6 +99,19 @@
   body {
     @apply bg-background text-foreground font-sans;
   }
+
+  /* Collapse all animation/transition to a single frame for users who ask the
+     OS for reduced motion. Covers the kit's data-open/closed animations, the
+     skeleton pulse, and the sidebar width transition in one place. */
+  @media (prefers-reduced-motion: reduce) {
+    *,
+    ::before,
+    ::after {
+      animation-duration: 0.01ms !important;
+      animation-iteration-count: 1 !important;
+      transition-duration: 0.01ms !important;
+    }
+  }
 }
 
 @layer components {