AccountMenu: add social login pending/error feedback and lock both providers during auth

[Copilot]

GitHub/Google login from AccountMenu could appear unresponsive during slow auth redirects, with no user feedback and no failure signal. This change adds explicit pending/error UX so users can distinguish “in progress” from “failed”.

• Pending state + mutual button lock

  • Added local signingInProvider state ("github" | "google" | null).
  • Clicking either social login button now disables both buttons until auth resolves/fails.
  • The clicked provider button shows an inline spinner (loading-spinner) while pending.

• Centralized social sign-in handler

  • Introduced signInWithSocial(provider) to unify callback URL construction and pending-state handling.
  • Guards against duplicate clicks while a sign-in is already in progress.

• Failure feedback

  • Added signinErrorMessage state.
  • On social auth failure, pending state is cleared and an inline Japanese error message is rendered in the menu.

<button
  onClick={() => signInWithSocial("github")}
  disabled={signingInProvider !== null}
  aria-busy={signingInProvider === "github"}
>
  {signingInProvider === "github" && (
    <span className="loading loading-spinner loading-xs" />
  )}
  GitHub でログイン
</button>

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	my-code	9c5cad5	Commit Preview URL Branch Preview URL	Jul 21 2026, 05:00 AM