fix(desktop): require active provider before Continue in setup step

Changed check from availableProviders.length > 0 to current?.available,
so Continue is only enabled when a provider is both configured and
selected as the active default.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/desktop/src/renderer/src/pages/onboarding/setup.tsx

@@ -23,7 +23,7 @@ export default function SetupStep() {
   const [focusedProvider, setFocusedProvider] =
     useState<ProviderStatus | null>(null)
 
-  const hasConfiguredProvider = availableProviders.length > 0
+  const hasActiveProvider = current?.available === true
 
   const handleConfigure = (provider: ProviderStatus) => {
     setSelectedProvider(provider)
@@ -91,7 +91,7 @@ export default function SetupStep() {
             <Button
               size="lg"
               onClick={handleContinue}
-              disabled={!hasConfiguredProvider}
+              disabled={!hasActiveProvider}
             >
               Continue
             </Button>