fix: restore mobile chat scrolling by fixing height constraint chain

Add overflow-hidden to MobileScreenStack stacked screen wrapper to
establish block formatting context. Add min-h-0 to MobileChatScreen
messages wrapper to override flex min-height: auto default.

packages/web/src/components/layout/MobileChatScreen.tsx

@@ -74,7 +74,7 @@ export function MobileChatScreen({ params }: MobileChatScreenProps) {
       </header>
 
       {/* Messages */}
-      <div className="flex-1 overflow-hidden">
+      <div className="flex-1 overflow-hidden min-h-0">
         {channelId && <MessageList channelId={channelId} />}
       </div>
 

packages/web/src/components/layout/MobileScreenStack.tsx

@@ -63,7 +63,7 @@ export function MobileScreenStack({ rootScreen, screenMap }: MobileScreenStackPr
         if (!renderer) return null;
 
         let style: React.CSSProperties = {};
-        let className = 'absolute inset-0 bg-surface-base z-10 flex flex-col';
+        let className = 'absolute inset-0 bg-surface-base z-10 flex flex-col overflow-hidden';
 
         if (isTop && transitioning === 'push') {
           // Initial position: off-screen right