feat(ui): improve mobile onboarding copy and chat input sizing

Rewrite user-facing text to be source-agnostic and result-focused:
- Connect prompt: "Scan to start" / "Scan a QR code to use an Agent"
- Empty chat: "Your Agent is ready"
- Input placeholder: "Ask your Agent..." / "Scan QR code to get started"
- Bump chat input font to text-base for mobile readability

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/ui/src/components/chat-input.tsx

@@ -50,7 +50,7 @@ export const ChatInput = forwardRef<ChatInputRef, ChatInputProps>(
       editorProps: {
         attributes: {
           class:
-            "w-full resize-none bg-transparent px-1 py-1 text-sm text-foreground outline-none placeholder:text-muted-foreground disabled:cursor-not-allowed",
+            "w-full resize-none bg-transparent px-1 py-1 text-base text-foreground outline-none placeholder:text-muted-foreground disabled:cursor-not-allowed",
         },
         handleKeyDown(_view, event) {
           // Guard for IME composition (Chinese/Japanese input)

packages/ui/src/components/chat.tsx

@@ -59,7 +59,7 @@ export function Chat() {
           <ConnectPrompt />
         ) : messages.length === 0 ? (
           <div className="flex items-center justify-center h-full text-muted-foreground text-sm">
-            Send a message to start the conversation
+            Your Agent is ready
           </div>
         ) : (
           <MessageList messages={messages} streamingIds={streamingIds} />
@@ -88,7 +88,7 @@ export function Chat() {
         <ChatInput
           onSubmit={handleSend}
           disabled={!isConnected}
-          placeholder={!isConnected ? "Connect first..." : "Type a message..."}
+          placeholder={!isConnected ? "Scan QR code to get started" : "Ask your Agent..."}
         />
       </footer>
     </div>