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/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
 		}