Fix mobile chat input positioning with dynamic viewport height

Use h-dvh instead of h-screen to account for mobile browser's dynamic address bar, keeping chat input always visible at bottom of viewport.

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

src/app/(session)/chat/page.tsx

@@ -70,7 +70,7 @@ export default function ChatPage() {
     };
 
     return (
-        <div className="flex flex-col h-screen bg-background">
+        <div className="flex flex-col h-dvh bg-background">
             {/* Session Header */}
             <div className="flex items-center justify-between px-4 py-3 bg-white border-b border-slate-200 shrink-0">
                 <div className="flex items-center gap-3">