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/routes/me.py

@@ -118,6 +118,12 @@ def get_me(
         "role": user.role,
         "is_demo": user.is_demo,
         "has_google": user.google_sub is not None,
+        # The address Google answers with, when it DIFFERS from the account's own. Same string for
+        # most people, and then this is None so no surface has to decide whether to show it — the
+        # comparison lives here, once, instead of in every consumer.
+        "google_email": (
+            user.google_email if user.google_email and user.google_email != user.email else None
+        ),
         "has_password": user.password_hash is not None,
         # Instance-wide: whether Google sign-in / YouTube connect is available at all (the UI hides
         # YouTube-access affordances and the onboarding nudge when it isn't configured).