fix(ads): restore reliable banner fades + keep banner on profile update

Two regressions from the previous banner pass:

- Fade (#2): the manual-opacity fade could paint opacity 0 and 1 in one frame and
  skip the transition — most visible for a single (default) campaign message,
  whose only fade is the first show. Revert the fade to Svelte transition:fade
  (which forces the from-state, so even the first/only message fades), keeping it
  on its own {#if} layer independent of the scroll. A freshly-mounted view onto a
  running cycle still renders the live message instantly (inFade duration 0 once),
  so navigation does not replay the fade. Verified by opacity sampling: advances
  fade, navigation stays at opacity 1.

- Profile update (#3): the banner block was attached only to GET /profile, so a
  profile.update (e.g. a language switch) returned a profile without it and the
  banner vanished until reload. A shared profileResponse() now attaches the banner
  to GET, PUT and the link/merge profile responses. Regression test added
  (TestBannerSurvivesProfileUpdate).

Still open: the scroll position is not preserved across navigation (the view
remounts); discussed separately.

backend/internal/server/handlers_link.go

@@ -180,7 +180,7 @@ func (s *Server) profileFor(ctx context.Context, id uuid.UUID) *profileResponse
 	if err != nil {
 		return nil
 	}
-	p := profileResponseFor(acc)
+	p := s.profileResponse(ctx, acc)
 	return &p
 }