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/banner.go

@@ -38,6 +38,16 @@ type bannerTimingsDTO struct {
 	FadeInMs       int `json:"fade_in_ms"`
 }
 
+// profileResponse builds the account's profile DTO with the advertising-banner block attached when
+// the viewer is eligible. Every endpoint returning the caller's own profile (get, update, link)
+// uses it, so the banner never drops off the client's profile after a non-get profile change (e.g.
+// a language switch).
+func (s *Server) profileResponse(ctx context.Context, acc account.Account) profileResponse {
+	r := profileResponseFor(acc)
+	r.Banner = s.bannerFor(ctx, acc)
+	return r
+}
+
 // bannerFor builds the advertising-banner block for the account's profile, or
 // nil when the ads service is not configured or the viewer is not eligible to
 // see a banner. The message language follows the account's bot (service)