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-issue.ts

@@ -1,3 +1,4 @@
+import {errorMessage} from '../modules/errors.ts';
 import {htmlEscape} from '../utils/html.ts';
 import {createTippy} from '../modules/tippy.ts';
 import {
@@ -425,7 +426,7 @@ export function initRepoIssueTitleEdit() {
       window.location.reload();
     } catch (error) {
       console.error(error);
-      showErrorToast(error.message);
+      showErrorToast(errorMessage(error));
     }
   });
 }