feat(i18n): share lobby message catalog across frontend/backend

frontend/tests/vertical-slice.test.ts

@@ -161,7 +161,7 @@ describe('vertical slice controller: lobby -> join -> start round', () => {
       joinSession: vi.fn().mockResolvedValue({
         ok: false,
         status: 404,
-        error: { kind: 'http', status: 404, message: 'HTTP 404', payload: { error: 'Session not found' } }
+        error: { kind: 'http', status: 404, message: 'HTTP 404', payload: { error: 'Session not found', error_code: 'session_not_found' } }
       })
     });
 
@@ -170,7 +170,7 @@ describe('vertical slice controller: lobby -> join -> start round', () => {
 
     const state = controller.getState();
     expect(state.joinState).toBe('error');
-    expect(state.errorMessage).toContain('Join fejlede');
+    expect(state.errorMessage).toBe('Session code is invalid or the session no longer exists.');
   });
 
   it('surfaces a friendly error when round start fails', async () => {
@@ -178,7 +178,7 @@ describe('vertical slice controller: lobby -> join -> start round', () => {
       startRound: vi.fn().mockResolvedValue({
         ok: false,
         status: 400,
-        error: { kind: 'http', status: 400, message: 'HTTP 400', payload: { error: 'Round can only be started from lobby' } }
+        error: { kind: 'http', status: 400, message: 'HTTP 400', payload: { error: 'Round can only be started from lobby', error_code: 'round_start_invalid_phase' } }
       })
     });
 
@@ -187,7 +187,7 @@ describe('vertical slice controller: lobby -> join -> start round', () => {
 
     const state = controller.getState();
     expect(state.startRoundState).toBe('error');
-    expect(state.errorMessage).toContain('Kunne ikke starte runden');
+    expect(state.errorMessage).toBe('Could not start round. Refresh the lobby and try again.');
   });
 
   it('shows local validation error and avoids API call when hydrating without any session code', async () => {
@@ -197,7 +197,7 @@ describe('vertical slice controller: lobby -> join -> start round', () => {
     await controller.hydrateLobby('   ');
 
     const state = controller.getState();
-    expect(state.errorMessage).toBe('Session-kode mangler.');
+    expect(state.errorMessage).toBe('Session code is required.');
     expect(state.loadingSession).toBe(false);
     expect(api.getSession).not.toHaveBeenCalled();
   });
@@ -210,7 +210,7 @@ describe('vertical slice controller: lobby -> join -> start round', () => {
 
     const state = controller.getState();
     expect(state.startRoundState).toBe('error');
-    expect(state.errorMessage).toBe('Session-kode mangler.');
+    expect(state.errorMessage).toBe('Session code is required.');
     expect(api.startRound).not.toHaveBeenCalled();
   });