feat(issue-226): add shared backend-frontend key-map and locale contract

frontend/tests/lobby-i18n.contract.test.ts

@@ -14,10 +14,16 @@ describe('shared i18n keyspace contract', () => {
     }
   });
 
-  it('keeps backend error-code keyspace aligned with backend translations', () => {
-    for (const [code, key] of Object.entries(lobbyCatalog.backend.error_codes)) {
-      expect(code).toBe(key);
-      expect(lobbyCatalog.backend.errors[key as keyof typeof lobbyCatalog.backend.errors]).toBeDefined();
+  it('keeps backend error-code keyspace aligned with shared backend→frontend map and backend translations', () => {
+    for (const [code, backendKey] of Object.entries(lobbyCatalog.backend.error_codes)) {
+      const frontendKey =
+        lobbyCatalog.contract.backend_to_frontend_error_keys[
+          code as keyof typeof lobbyCatalog.contract.backend_to_frontend_error_keys
+        ];
+
+      expect(lobbyCatalog.backend.errors[backendKey as keyof typeof lobbyCatalog.backend.errors]).toBeDefined();
+      expect(frontendKey, `missing frontend mapping for ${code}`).toBeTruthy();
+      expect(lobbyCatalog.frontend.errors[frontendKey as keyof typeof lobbyCatalog.frontend.errors]).toBeDefined();
     }
 
     for (const [key, translations] of Object.entries(lobbyCatalog.backend.errors)) {
@@ -42,7 +48,7 @@ describe('lobbyMessage locale handling', () => {
     ).toBe('Sessionskoden er ugyldig, eller sessionen findes ikke længere.');
   });
 
-  it('falls back when backend error code has no frontend translation key', () => {
+  it('uses shared backend→frontend key-map when backend key differs from frontend copy key', () => {
     expect(
       lobbyMessageFromApiPayload(
         { error_code: 'session_not_joinable', locale: 'da' },
@@ -50,4 +56,13 @@ describe('lobbyMessage locale handling', () => {
       ),
     ).toBe('Kunne ikke joine. Tjek kode eller kaldenavn og prøv igen.');
   });
+
+  it('falls back to caller-provided fallback key for unknown backend error codes', () => {
+    expect(
+      lobbyMessageFromApiPayload(
+        { error_code: 'unknown_backend_key', locale: 'da' },
+        'join_failed',
+      ),
+    ).toBe('Kunne ikke joine. Tjek kode eller kaldenavn og prøv igen.');
+  });
 });