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/markup/common.ts

@@ -1,8 +1,10 @@
-export function displayError(el: Element, err: Error): void {
+import {errorMessage} from '../modules/errors.ts';
+
+export function displayError(el: Element, err: unknown): void {
   el.classList.remove('is-loading');
   const errorNode = document.createElement('pre');
   errorNode.setAttribute('class', 'ui message error markup-block-error');
-  errorNode.textContent = err.message || String(err);
+  errorNode.textContent = errorMessage(err);
   el.before(errorNode);
   el.setAttribute('data-render-done', 'true');
 }