feat(auth): improve error handling for popup-blocked sign-in attempts

frontend/src/auth/AuthProvider.tsx

@@ -78,17 +78,12 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
             try {
                 await signInWithPopup(auth, provider);
             } catch (err: any) {
+                // Avoid redirect fallback to prevent navigation/404; surface a clearer popup-blocked message.
                 const code = err?.code ?? "";
-                // Common cases where redirect works better
-                const redirectable = [
-                    "auth/popup-blocked",
-                    "auth/operation-not-supported-in-this-environment",
-                    "auth/unauthorized-domain",
-                ];
-                if (redirectable.includes(code)) {
-                    const { signInWithRedirect } = await import("firebase/auth");
-                    await signInWithRedirect(auth, provider);
-                    return; // navigation will happen
+                if (code === "auth/popup-blocked") {
+                    setError("ポップアップがブロックされました。ブラウザで許可してください。");
+                    console.error("signin popup blocked:", err);
+                    return;
                 }
                 const msg = err instanceof Error ? err.message : String(err);
                 setError(msg);               // keep the message for the UI