fix: permissions matrix UI now returns proper shape from backend

The frontend admin.js expects {sets, users, permissions} but
ListPermissions returned a flat []SetPermission slice.

- Add PermissionsMatrix struct with JSON tags
- Update AdminService.ListPermissions to return *PermissionsMatrix
- Update mocks and all call sites to match new signature
- Update tests to assert the new matrix shape

1 files changed, 5 insertions, 2 deletions

diff --git a/internal/service/admin_test.go b/internal/service/admin_test.go
index d6dbfeb..afacc3a 100644
--- a/internal/service/admin_test.go
+++ b/internal/service/admin_test.go
@@ -205,8 +205,11 @@ func TestAdminService_ListPermissions(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
-	if len(perms) != 2 {
-		t.Fatalf("expected 2 permissions, got %d", len(perms))
+	if perms == nil || len(perms.Permissions) != 2 {
+		t.Fatalf("expected 2 permissions, got %+v", perms)
+	}
+	if len(perms.Sets) != 2 {
+		t.Fatalf("expected 2 sets, got %d", len(perms.Sets))
 	}
 }