fix(account): seed a fresh guest's interface language from the client locale

A guest was provisioned with only its detected time zone; its PreferredLanguage
fell to the 'en' column default. The client already sends its locale in the guest
login (GuestLoginRequest.locale), but the gateway dropped it and ProvisionGuest
took no language — so a Russian user's brand-new native guest got English
language-dependent server content (the ad banner, bot messages) until the client's
later language reconcile. Read the locale in the gateway guest handler, thread it
through GuestAuth -> ProvisionGuest, and seed PreferredLanguage from it (validated;
an unsupported/absent value keeps the 'en' default). Wire field already present —
no schema change.

gateway/internal/backendclient/api.go

@@ -312,11 +312,12 @@ func (c *Client) ChatAccessByUser(ctx context.Context, userID string) (ChatAcces
 }
 
 // GuestAuth provisions a guest account and mints a session, seeding its time zone
-// from browserTz (the client's detected "±HH:MM" UTC offset).
-func (c *Client) GuestAuth(ctx context.Context, browserTz, subtype string) (SessionResp, error) {
+// from browserTz (the client's detected "±HH:MM" UTC offset) and its interface
+// language from language (the client's detected locale; best-effort).
+func (c *Client) GuestAuth(ctx context.Context, browserTz, subtype, language string) (SessionResp, error) {
 	var out SessionResp
 	err := c.do(ctx, http.MethodPost, "/api/v1/internal/sessions/guest", "", "",
-		map[string]string{"browser_tz": browserTz, "subtype": subtype}, &out)
+		map[string]string{"browser_tz": browserTz, "subtype": subtype, "language": language}, &out)
 	return out, err
 }
 

gateway/internal/transcode/transcode.go

@@ -270,13 +270,14 @@ func authGuestHandler(backend *backendclient.Client) Handler {
 		// The guest bootstrap historically carried no payload; the detected zone is
 		// optional, so an absent or empty one simply yields no time-zone seed (rather
 		// than panicking in GetRootAs* on a zero-length buffer).
-		var browserTz, subtype string
+		var browserTz, subtype, locale string
 		if len(req.Payload) > 0 {
 			g := fb.GetRootAsGuestLoginRequest(req.Payload, 0)
 			browserTz = string(g.BrowserTz())
 			subtype = string(g.Subtype())
+			locale = string(g.Locale())
 		}
-		sess, err := backend.GuestAuth(ctx, browserTz, subtype)
+		sess, err := backend.GuestAuth(ctx, browserTz, subtype, locale)
 		if err != nil {
 			return nil, err
 		}