Enable strict TypeScript, add errorMessage helper (#37292)

Enable full TypeScript `strict` mode and fix issues discovered during
this refactor. Introduced a `errorMessage` helper function to cleanly
extract a error messages from the `unknown` type.

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (claude-opus-4-7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/repo-settings-branches.ts

@@ -2,6 +2,7 @@ import {createSortable} from '../modules/sortable.ts';
 import {POST} from '../modules/fetch.ts';
 import {showErrorToast} from '../modules/toast.ts';
 import {queryElemChildren} from '../utils/dom.ts';
+import {errorMessage} from '../modules/errors.ts';
 
 export function initRepoSettingsBranchesDrag() {
   const protectedBranchesList = document.querySelector<HTMLElement>('#protected-branches-list');
@@ -23,8 +24,7 @@ export function initRepoSettingsBranchesDrag() {
             },
           });
         } catch (err) {
-          const errorMessage = String(err);
-          showErrorToast(`Failed to update branch protection rule priority:, error: ${errorMessage}`);
+          showErrorToast(`Failed to update branch protection rule priority: ${errorMessage(err)}`);
         }
       })();
     },