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/components/ui/select.tsx

@@ -99,7 +99,7 @@ function SelectContent({
         <SelectPrimitive.Popup
           data-slot="select-content"
           className={cn(
-            "max-h-[min(24rem,var(--available-height))] min-w-[var(--anchor-width)] overflow-y-auto rounded-md border bg-popover p-1 text-popover-foreground shadow-md outline-none",
+            "max-h-[min(24rem,var(--available-height))] min-w-[var(--anchor-width)] overflow-y-auto overscroll-y-contain rounded-md border bg-popover p-1 text-popover-foreground shadow-md outline-none",
             "data-open:animate-in data-open:fade-in-0 data-closed:animate-out data-closed:fade-out-0",
             className
           )}