fix(walkthrough): hide unauthenticated models and disable Generate

Do not present a provider without a login as the selected walkthrough
model, and grey out Generate when readiness is false instead of showing
a login-error blocker or raw auth banner.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/ui/src/lib/walkthrough/types.ts

@@ -105,6 +105,8 @@ export interface WalkthroughReadiness {
     inputCharBudget?: number;
     contextTokens?: number;
     structuredOutput?: boolean | null;
+    /** False when the resolved provider has no usable OpenCode login. */
+    hasLogin?: boolean;
   };
   requiredChars?: number;
   availableChars?: number;