feat(auth): show the Google address beside the one the account registered with (E9.5 S3)

An account can now hold two true addresses: the one its owner typed, which a Google
sign-in no longer overwrites, and the one Google answers with. Only the first was
stored, so Settings could say 'Connected' without saying connected AS WHOM -- exactly
what someone who registered with a +tag needs to know.

users.google_email (migration 0080) records Google's answer at sign-in and at an
in-app link. /api/me exposes it ONLY when it differs from the account address, so the
comparison lives in one place and the majority, whose two addresses are the same
string, see no change at all. Rendered in the account menu as a muted third line under
the identity, and in the Settings sign-in row under the existing hint.

A short 'Google' label rather than a brand glyph: the app draws one icon set (lucide),
and a generic mail icon next to an email address says nothing. It also reads to a
screen reader, which an unlabelled logo would not.

No backfill is possible -- Google's answer is only known at sign-in -- so accounts
linked earlier stay NULL until their owner next signs in with Google.

frontend/src/components/SettingsPanel.tsx

@@ -413,6 +413,15 @@ function SignInMethods({ me }: { me: Me }) {
                 ? t("settings.account.googleLink.connectedHint")
                 : t("settings.account.googleLink.connectHint")}
             </p>
+            {/* Which address Google actually answers with — sent only when it differs from the
+                account's own (see /api/me). "Connected" alone left that unanswerable, and it is
+                exactly what someone who registered with a `+tag` needs to know. */}
+            {me.google_email && (
+              <p className="text-xs text-fg/80 mt-1.5">
+                <span className="text-muted">{t("settings.account.googleLink.signedInAs")}</span>{" "}
+                {me.google_email}
+              </p>
+            )}
           </div>
           {me.has_google ? (
             <span className="shrink-0 text-2xs px-2 py-1 rounded-full border border-accent/40 text-accent">