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)
+	}
+}

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)

backend/internal/server/handlers_account.go

@@ -83,7 +83,7 @@ func (s *Server) handleUpdateProfile(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
-	c.JSON(http.StatusOK, profileResponseFor(acc))
+	c.JSON(http.StatusOK, s.profileResponse(c.Request.Context(), acc))
 }
 
 // blockStatusResponse reports the caller's current block to the client. Until is an RFC3339 UTC

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
 }
 

backend/internal/server/handlers_user.go

@@ -23,9 +23,7 @@ func (s *Server) handleProfile(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
-	resp := profileResponseFor(acc)
-	resp.Banner = s.bannerFor(c.Request.Context(), acc)
-	c.JSON(http.StatusOK, resp)
+	c.JSON(http.StatusOK, s.profileResponse(c.Request.Context(), acc))
 }
 
 // submitPlayRequest places tiles on the player's turn; the engine infers the