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/inttest/banner_console_test.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"encoding/json"
 	"net/http"
+	"net/http/httptest"
 	"strings"
 	"testing"
 
@@ -247,3 +248,28 @@ func TestBannerProfileEligibility(t *testing.T) {
 		t.Fatal("a non-empty hint wallet still shows the banner")
 	}
 }
+
+// TestBannerSurvivesProfileUpdate guards that a profile update (e.g. a language switch) returns the
+// banner block too, so the client's profile keeps the banner instead of losing it until reload.
+func TestBannerSurvivesProfileUpdate(t *testing.T) {
+	srv, _ := bannerServer(t)
+	id := provisionAccount(t)
+
+	body := `{"display_name":"Tester","preferred_language":"ru","time_zone":"UTC","away_start":"00:00",` +
+		`"away_end":"00:00","block_chat":false,"block_friend_requests":false,"notifications_in_app_only":true}`
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodPut, "/api/v1/user/profile", strings.NewReader(body))
+	req.Header.Set("X-User-ID", id.String())
+	req.Header.Set("Content-Type", "application/json")
+	srv.Handler().ServeHTTP(rec, req)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("update profile = %d: %s", rec.Code, rec.Body.String())
+	}
+	var p profileBanner
+	if err := json.Unmarshal(rec.Body.Bytes(), &p); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if p.Banner == nil || len(p.Banner.Campaigns) == 0 {
+		t.Fatalf("profile update dropped the banner block: %v", p.Banner)
+	}
+}