feat(session): carry the bot service_language on the Session wire

Thread the Telegram bot's service language (en/ru) from the session mint response
through the gateway into the FlatBuffers Session, so the UI knows which bot the
player signed in through. handleTelegramAuth refreshes the account's service
language onto the response before minting (it was set after the fetched copy).
Empty for a non-Telegram login.

backend/internal/server/dto.go

@@ -16,10 +16,11 @@ import (
 
 // sessionResponse is the credential returned by every auth endpoint.
 type sessionResponse struct {
-	Token       string `json:"token"`
-	UserID      string `json:"user_id"`
-	IsGuest     bool   `json:"is_guest"`
-	DisplayName string `json:"display_name"`
+	Token           string `json:"token"`
+	UserID          string `json:"user_id"`
+	IsGuest         bool   `json:"is_guest"`
+	DisplayName     string `json:"display_name"`
+	ServiceLanguage string `json:"service_language"`
 }
 
 // okResponse is a simple success acknowledgement.
@@ -159,10 +160,11 @@ type errorBody struct {
 // sessionResponseFor builds the credential payload for a minted session.
 func sessionResponseFor(token string, acc account.Account) sessionResponse {
 	return sessionResponse{
-		Token:       token,
-		UserID:      acc.ID.String(),
-		IsGuest:     acc.IsGuest,
-		DisplayName: acc.DisplayName,
+		Token:           token,
+		UserID:          acc.ID.String(),
+		IsGuest:         acc.IsGuest,
+		DisplayName:     acc.DisplayName,
+		ServiceLanguage: acc.ServiceLanguage,
 	}
 }
 

backend/internal/server/handlers_auth.go

@@ -48,6 +48,7 @@ func (s *Server) handleTelegramAuth(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
+	acc.ServiceLanguage = req.ServiceLanguage // reflect this login's bot in the session response
 	s.mintSession(c, acc)
 }
 

gateway/internal/backendclient/api.go

@@ -14,10 +14,11 @@ import (
 
 // SessionResp is the credential minted by an auth operation.
 type SessionResp struct {
-	Token       string `json:"token"`
-	UserID      string `json:"user_id"`
-	IsGuest     bool   `json:"is_guest"`
-	DisplayName string `json:"display_name"`
+	Token           string `json:"token"`
+	UserID          string `json:"user_id"`
+	IsGuest         bool   `json:"is_guest"`
+	DisplayName     string `json:"display_name"`
+	ServiceLanguage string `json:"service_language"`
 }
 
 // ProfileResp is an account's own profile.

gateway/internal/transcode/encode.go

@@ -36,6 +36,7 @@ func encodeSession(s backendclient.SessionResp, supportedLangs []string) []byte
 	token := b.CreateString(s.Token)
 	uid := b.CreateString(s.UserID)
 	name := b.CreateString(s.DisplayName)
+	svcLang := b.CreateString(s.ServiceLanguage)
 	langs := buildSupportedLanguagesVector(b, supportedLangs)
 	fb.SessionStart(b)
 	fb.SessionAddToken(b, token)
@@ -43,6 +44,7 @@ func encodeSession(s backendclient.SessionResp, supportedLangs []string) []byte
 	fb.SessionAddIsGuest(b, s.IsGuest)
 	fb.SessionAddDisplayName(b, name)
 	fb.SessionAddSupportedLanguages(b, langs)
+	fb.SessionAddServiceLanguage(b, svcLang)
 	b.Finish(fb.SessionEnd(b))
 	return b.FinishedBytes()
 }

gateway/internal/transcode/transcode_telegram_test.go

@@ -43,7 +43,7 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
 		_ = json.NewDecoder(r.Body).Decode(&gotBody)
-		_, _ = w.Write([]byte(`{"token":"tok-tg","user_id":"u-tg","is_guest":false,"display_name":"Иван"}`))
+		_, _ = w.Write([]byte(`{"token":"tok-tg","user_id":"u-tg","is_guest":false,"display_name":"Иван","service_language":"ru"}`))
 	})
 	defer cleanup()
 
@@ -67,6 +67,11 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 	if got := sessionLanguages(sess); len(got) != 1 || got[0] != "ru" {
 		t.Errorf("session supported_languages = %v, want [ru]", got)
 	}
+	// The bot's service language rides the Session so the UI can build a share link to
+	// the same bot.
+	if got := string(sess.ServiceLanguage()); got != "ru" {
+		t.Errorf("session service_language = %q, want ru", got)
+	}
 	// The validated launch fields are forwarded so the backend can seed a new account;
 	// service_language is recorded to route the account's later out-of-app push.
 	if gotBody["external_id"] != "42" || gotBody["language_code"] != "ru" || gotBody["first_name"] != "Иван" || gotBody["service_language"] != "ru" {

pkg/fbs/scrabble.fbs

@@ -123,6 +123,10 @@ table Session {
   is_guest:bool;
   display_name:string;
   supported_languages:[string];
+  // service_language is the language tag (en/ru) of the Telegram bot the session was
+  // minted through; empty for a non-Telegram login. The UI uses it to build a share
+  // link that points at the same bot.
+  service_language:string;
 }
 
 // Ack is a simple success acknowledgement (e.g. an email-code request).

pkg/fbs/scrabblefb/Session.go

@@ -94,8 +94,16 @@ func (rcv *Session) SupportedLanguagesLength() int {
 	return 0
 }
 
+func (rcv *Session) ServiceLanguage() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(14))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
 func SessionStart(builder *flatbuffers.Builder) {
-	builder.StartObject(5)
+	builder.StartObject(6)
 }
 func SessionAddToken(builder *flatbuffers.Builder, token flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(token), 0)
@@ -115,6 +123,9 @@ func SessionAddSupportedLanguages(builder *flatbuffers.Builder, supportedLanguag
 func SessionStartSupportedLanguagesVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
 	return builder.StartVector(4, numElems, 4)
 }
+func SessionAddServiceLanguage(builder *flatbuffers.Builder, serviceLanguage flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(5, flatbuffers.UOffsetT(serviceLanguage), 0)
+}
 func SessionEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }

ui/src/gen/fbs/scrabblefb/session.ts

@@ -58,8 +58,15 @@ supportedLanguagesLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+serviceLanguage():string|null
+serviceLanguage(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+serviceLanguage(optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 14);
+  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+}
+
 static startSession(builder:flatbuffers.Builder) {
-  builder.startObject(5);
+  builder.startObject(6);
 }
 
 static addToken(builder:flatbuffers.Builder, tokenOffset:flatbuffers.Offset) {
@@ -94,18 +101,23 @@ static startSupportedLanguagesVector(builder:flatbuffers.Builder, numElems:numbe
   builder.startVector(4, numElems, 4);
 }
 
+static addServiceLanguage(builder:flatbuffers.Builder, serviceLanguageOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(5, serviceLanguageOffset, 0);
+}
+
 static endSession(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createSession(builder:flatbuffers.Builder, tokenOffset:flatbuffers.Offset, userIdOffset:flatbuffers.Offset, isGuest:boolean, displayNameOffset:flatbuffers.Offset, supportedLanguagesOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createSession(builder:flatbuffers.Builder, tokenOffset:flatbuffers.Offset, userIdOffset:flatbuffers.Offset, isGuest:boolean, displayNameOffset:flatbuffers.Offset, supportedLanguagesOffset:flatbuffers.Offset, serviceLanguageOffset:flatbuffers.Offset):flatbuffers.Offset {
   Session.startSession(builder);
   Session.addToken(builder, tokenOffset);
   Session.addUserId(builder, userIdOffset);
   Session.addIsGuest(builder, isGuest);
   Session.addDisplayName(builder, displayNameOffset);
   Session.addSupportedLanguages(builder, supportedLanguagesOffset);
+  Session.addServiceLanguage(builder, serviceLanguageOffset);
   return Session.endSession(builder);
 }
 }

ui/src/lib/codec.test.ts

@@ -171,6 +171,7 @@ describe('codec', () => {
       isGuest: true,
       displayName: 'Me',
       supportedLanguages: ['en', 'ru'],
+      serviceLanguage: '',
     });
   });
 
@@ -311,7 +312,7 @@ describe('codec', () => {
     b.finish(fb.LinkResult.endLinkResult(b));
     const r = decodeLinkResult(b.asUint8Array());
     expect(r.status).toBe('merged');
-    expect(r.session).toEqual({ token: 'tok-9', userId: 'a-1', isGuest: false, displayName: 'Kaya', supportedLanguages: [] });
+    expect(r.session).toEqual({ token: 'tok-9', userId: 'a-1', isGuest: false, displayName: 'Kaya', supportedLanguages: [], serviceLanguage: '' });
   });
 
   it('decodes an Invitation with inviter and invitees', () => {

ui/src/lib/codec.ts

@@ -292,6 +292,7 @@ function sessionFromTable(t: fb.Session): Session {
     isGuest: t.isGuest(),
     displayName: s(t.displayName()),
     supportedLanguages,
+    serviceLanguage: s(t.serviceLanguage()),
   };
 }
 

ui/src/lib/mock/data.ts

@@ -25,6 +25,7 @@ export const SESSION: Session = {
   displayName: 'You',
   // Both languages by default, so the mock-driven UI offers every variant.
   supportedLanguages: ['en', 'ru'],
+  serviceLanguage: '',
 };
 
 export const PROFILE: Profile = {

ui/src/lib/model.ts

@@ -234,6 +234,9 @@ export interface Session {
   // variants these languages support (en -> English; ru -> Russian + Эрудит). Empty
   // means ungated (all variants).
   supportedLanguages: string[];
+  // serviceLanguage is the en/ru tag of the Telegram bot this session was minted
+  // through (empty for a non-Telegram login); used to share a link to the same bot.
+  serviceLanguage: string;
 }
 
 // LinkResult is the outcome of an account link/merge step. status is