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/modules/errors.ts

@@ -2,6 +2,10 @@
 import {html} from '../utils/html.ts';
 import type {Intent} from '../types.ts';
 
+export function errorMessage(err: unknown): string {
+  return (err as Error)?.message || String(err);
+}
+
 export function showGlobalErrorMessage(msg: string, msgType: Intent = 'error') {
   const msgContainer = document.querySelector('.page-content') ?? document.body;
   if (!msgContainer) {