fix(onboarding): wire header buttons to v2 inline auth flow

Login opens the auth modal with login display, signup opens the signup
chooser. Both use the v2 component's own auth flow instead of
showLogin which redirects to /onboarding.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: capJavert

packages/webapp/components/onboarding/OnboardingV2.tsx

@@ -1143,18 +1143,21 @@ export const OnboardingV2 = (): ReactElement => {
           position={LogoPosition.Relative}
           className="!left-0 !top-0 !mt-0 !translate-x-0"
         />
-        <div className="ml-auto flex items-center gap-3">
+        <div className="ml-auto flex items-center gap-4">
           <button
             type="button"
-            onClick={openLogin}
-            className="rounded-12 px-3 py-1.5 font-bold text-text-secondary transition-colors duration-200 typo-callout hover:bg-surface-hover"
+            onClick={() => {
+              setAuthDisplay(AuthDisplay.Default);
+              setShowAuthSignup(true);
+            }}
+            className="h-10 rounded-14 border border-border-subtlest-tertiary px-5 font-bold text-text-primary transition-colors duration-200 typo-callout hover:bg-surface-hover"
           >
             Log in
           </button>
           <button
             type="button"
             onClick={() => setShowSignupChooser(true)}
-            className="hover:opacity-90 rounded-12 bg-accent-cabbage-default px-5 py-1.5 font-bold text-white transition-opacity duration-200 typo-callout"
+            className="h-10 rounded-14 bg-white px-5 font-bold text-black transition-opacity duration-200 typo-callout hover:opacity-90"
           >
             Sign up
           </button>