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.

backend/app/models.py

@@ -66,6 +66,12 @@ class User(Base, TimestampMixin, CanonicalEmailMixin):
     # NOT unique: prod already carries two rows for one mailbox (a password registration and the
     # Google sign-in that failed to find it), and a migration must not merge accounts on its own.
     email_canonical: Mapped[str] = mapped_column(String(320), index=True, server_default="")
+    # The address Google asserted for this account, recorded at sign-in / link time. NOT the same
+    # thing as `email`: since E9.5 S3 a Google sign-in no longer overwrites the address the user
+    # registered with, so a deliberate `me+siftlode@gmail.com` survives — and this is where the other
+    # half of the truth lives, for the UI to show when the two differ. NULL until the next Google
+    # sign-in for accounts linked before this column existed.
+    google_email: Mapped[str | None] = mapped_column(String(320))
     # argon2id hash for email+password login; NULL for Google-only / demo accounts.
     password_hash: Mapped[str | None] = mapped_column(String(255))
     # When SEC-10 dropped a preset password on Google adoption (see the single caller of