feat(ux): Finalize advanced UX with plot zoom, export, and shadcn UI consistency.

frontend/src/App.tsx

@@ -9,6 +9,7 @@ import { AuthService, type UserResponse } from "./services/auth"
 import { ChatService, type MessageResponse } from "./services/chat"
 import { type Conversation } from "./components/layout/HistorySidebar"
 import { registerUnauthorizedCallback } from "./services/api"
+import { Button } from "./components/ui/button"
 
 function App() {
   const [isAuthenticated, setIsAuthenticated] = useState(false)
@@ -179,12 +180,13 @@ function App() {
                       Welcome, {user?.display_name || user?.email || "User"}!
                     </h1>
                   </div>
-                  <button
+                  <Button
+                    variant="link"
                     onClick={handleLogout}
-                    className="text-sm text-muted-foreground hover:text-primary underline"
+                    className="text-sm text-muted-foreground hover:text-primary h-auto p-0 font-normal underline"
                   >
                     Logout
-                  </button>
+                  </Button>
                 </div>
                 
                 <div className="flex-1 min-h-0">
@@ -218,12 +220,12 @@ function App() {
                         <p className="text-sm text-muted-foreground">
                           Create a new conversation in the sidebar to start asking questions.
                         </p>
-                        <button 
+                        <Button 
                           onClick={handleCreateConversation}
-                          className="mt-4 px-4 py-2 bg-primary text-primary-foreground rounded-md text-sm font-medium hover:bg-primary/90"
+                          className="mt-4"
                         >
                           Start New Chat
-                        </button>
+                        </Button>
                       </div>
                     </div>
                   )}