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/alembic/versions/0080_google_email.py

@@ -0,0 +1,32 @@
+"""users.google_email — the address Google asserts, kept beside the one the user registered with
+
+Revision ID: 0080_google_email
+Revises: 0079_email_canonical
+Create Date: 2026-09-04
+
+Since 0079's follow-up a Google sign-in no longer overwrites `users.email`, so a deliberate
+`me+siftlode@gmail.com` survives being linked to a Google account whose primary is `me@gmail.com`.
+That leaves two true addresses for one account and only one of them stored. This column holds the
+other, so the account menu and the Settings sign-in row can show which address Google actually
+answers with.
+
+No backfill is possible: Google's answer is only known at sign-in. Accounts linked before this
+column existed carry NULL until their owner next signs in with Google — and for the majority the two
+addresses are identical anyway, where the UI shows nothing either way.
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0080_google_email"
+down_revision: str | None = "0079_email_canonical"
+branch_labels: str | None = None
+depends_on: str | None = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("google_email", sa.String(length=320), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("users", "google_email")