fix: harden Firebase auth error handling and add config validation

frontend/src/components/auth/SocialAuth.jsx

@@ -1,5 +1,6 @@
 import { useState } from 'react';
 import { useAuth } from '../../context/AuthContext';
+import { firebaseConfigured } from '../../config/firebase.js';
 
 const GOOGLE_ICON = (
   <svg width="18" height="18" viewBox="0 0 24 24">
@@ -41,30 +42,36 @@ export default function SocialAuth() {
         <div className="flex-1 border-t border-surface-border" />
       </div>
 
-      {error && (
-        <p className="text-sm text-red-500 mb-3 text-center">{error}</p>
-      )}
+      {!firebaseConfigured ? (
+        <p className="text-xs text-text-tertiary text-center">Social sign-in is not configured.</p>
+      ) : (
+        <>
+          {error && (
+            <p className="text-sm text-red-500 mb-3 text-center">{error}</p>
+          )}
 
-      <div className="space-y-3">
-        <button
-          type="button"
-          onClick={() => handleProviderLogin('google')}
-          disabled={loadingProvider !== null}
-          className="w-full py-3 bg-surface text-text-primary text-sm font-medium rounded-lg border border-surface-border hover:bg-surface-secondary transition-colors flex items-center justify-center gap-3 disabled:opacity-60 disabled:cursor-not-allowed"
-        >
-          {GOOGLE_ICON}
-          {loadingProvider === 'google' ? 'Signing in...' : 'Continue with Google'}
-        </button>
-        <button
-          type="button"
-          onClick={() => handleProviderLogin('github')}
-          disabled={loadingProvider !== null}
-          className="w-full py-3 bg-surface text-text-primary text-sm font-medium rounded-lg border border-surface-border hover:bg-surface-secondary transition-colors flex items-center justify-center gap-3 disabled:opacity-60 disabled:cursor-not-allowed"
-        >
-          {GITHUB_ICON}
-          {loadingProvider === 'github' ? 'Signing in...' : 'Continue with GitHub'}
-        </button>
-      </div>
+          <div className="space-y-3">
+            <button
+              type="button"
+              onClick={() => handleProviderLogin('google')}
+              disabled={loadingProvider !== null}
+              className="w-full py-3 bg-surface text-text-primary text-sm font-medium rounded-lg border border-surface-border hover:bg-surface-secondary transition-colors flex items-center justify-center gap-3 disabled:opacity-60 disabled:cursor-not-allowed"
+            >
+              {GOOGLE_ICON}
+              {loadingProvider === 'google' ? 'Signing in...' : 'Continue with Google'}
+            </button>
+            <button
+              type="button"
+              onClick={() => handleProviderLogin('github')}
+              disabled={loadingProvider !== null}
+              className="w-full py-3 bg-surface text-text-primary text-sm font-medium rounded-lg border border-surface-border hover:bg-surface-secondary transition-colors flex items-center justify-center gap-3 disabled:opacity-60 disabled:cursor-not-allowed"
+            >
+              {GITHUB_ICON}
+              {loadingProvider === 'github' ? 'Signing in...' : 'Continue with GitHub'}
+            </button>
+          </div>
+        </>
+      )}
     </div>
   );
 }

frontend/src/config/firebase.js

@@ -3,11 +3,19 @@ import { getAuth, GoogleAuthProvider, GithubAuthProvider } from 'firebase/auth';
 
 const getMeta = (name) => document.querySelector(`meta[name="${name}"]`)?.content || '';
 
-const app = initializeApp({
+const firebaseConfig = {
   apiKey: import.meta.env.VITE_FIREBASE_API_KEY || getMeta('fb-api-key'),
   authDomain: import.meta.env.VITE_FIREBASE_AUTH_DOMAIN || getMeta('fb-auth-domain'),
   projectId: import.meta.env.VITE_FIREBASE_PROJECT_ID || getMeta('fb-project-id'),
-});
+};
+
+export const firebaseConfigured = !!(firebaseConfig.apiKey && firebaseConfig.authDomain && firebaseConfig.projectId);
+
+if (import.meta.env.DEV && !firebaseConfigured) {
+  console.warn('[Firebase] Missing env vars: VITE_FIREBASE_API_KEY, VITE_FIREBASE_AUTH_DOMAIN, VITE_FIREBASE_PROJECT_ID. Social sign-in will not work.');
+}
+
+const app = initializeApp(firebaseConfig);
 
 export const auth = getAuth(app);
 export const providers = {

frontend/src/context/AuthContext.jsx

@@ -70,13 +70,23 @@ export function AuthProvider({ children }) {
       dispatch({ type: 'AUTH_SUCCESS', payload: data.user });
     } catch (err) {
       await firebaseSignOut(auth).catch(() => {});
-      let message = err.message;
+      if (err.code === 'auth/cancelled-popup-request') {
+        dispatch({ type: 'LOADING' });
+        return;
+      }
+      let message = 'Sign-in failed. Please try again.';
       if (err.code === 'auth/account-exists-with-different-credential') {
         const other = providerName === 'google' ? 'GitHub' : 'Google';
         message = `An account with this email already exists. Try signing in with ${other} instead.`;
+      } else if (err.code === 'auth/popup-blocked') {
+        message = 'Sign-in popup was blocked by your browser. Please allow popups for this site and try again.';
+      } else if (err.code === 'auth/popup-closed-by-user') {
+        message = 'Sign-in window was closed. If this keeps happening, check that your browser allows popups for this site.';
+      } else if (err.code === 'auth/unauthorized-domain') {
+        message = 'This domain is not authorized for sign-in. Add the deployment URL to Firebase Console → Authentication → Authorized domains.';
       }
       dispatch({ type: 'AUTH_ERROR', payload: message });
-      throw err;
+      throw new Error(message);
     }
   }, []);