fix(walkthrough): block unauthenticated providers with a friendly refusal

When the walkthrough small model resolves to a provider with no usable
login, readiness was still ready and generate returned a raw 500 message.
Refuse up front with no-provider-login and surface a blocker instead.

Closes openchamber/openchamber#2607

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

packages/web/server/lib/walkthrough/index.js

@@ -333,6 +333,12 @@ function computeReadiness({ model, digest, files, fileCount, hunkCount, generate
     return { ready: false, reason, model, generatedFileCount };
   }
 
+  // A resolved override/config model can still have no usable login. Refuse up
+  // front so the panel does not look ready and then dump a raw auth error.
+  if (model.hasLogin === false) {
+    return { ready: false, reason: 'no-provider-login', model };
+  }
+
   // Built with the same language the generation would use: the instruction is
   // part of the prompt, so a readiness answer computed without it would be
   // measuring a request nobody is going to send.
@@ -392,6 +398,13 @@ async function runGeneration({ directory, source, repoRoot, key, force, explicit
   if (!model) {
     throw fail('No model is available — sign in to a provider first', 404, { code: 'no-model' });
   }
+  if (model.hasLogin === false) {
+    throw fail(
+      `No OpenCode login found for provider "${model.providerID}" — sign in or choose a different model`,
+      401,
+      { code: 'no-provider-login', model },
+    );
+  }
 
   const { digest, files, idByAlias, fileCount, hunkCount, generatedFileCount } = await loadCurrentDiff(directory, source, deps);
   setStage(repoRoot, key, 'asking');
@@ -494,6 +507,9 @@ async function runGeneration({ directory, source, repoRoot, key, force, explicit
     if (error?.code === 'output-exhausted') {
       return fail(error.message, 409, { code: 'output-exhausted', model });
     }
+    if (error?.code === 'no-provider-login') {
+      return fail(error.message, 401, { code: 'no-provider-login', model });
+    }
     return null;
   };