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-edit.ts

@@ -3,6 +3,7 @@ import {getComboMarkdownEditor, initComboMarkdownEditor, ComboMarkdownEditor} fr
 import {POST} from '../modules/fetch.ts';
 import {showErrorToast} from '../modules/toast.ts';
 import {hideElem, querySingleVisibleElem, showElem} from '../utils/dom.ts';
+import {errorMessage} from '../modules/errors.ts';
 import {triggerUploadStateChanged} from './comp/EditorUpload.ts';
 import {convertHtmlToMarkdown} from '../markup/html2markdown.ts';
 import {applyAreYouSure, reinitializeAreYouSure} from '../vendor/jquery.are-you-sure.ts';
@@ -73,7 +74,7 @@ async function tryOnEditContent(e: Event) {
       }
       comboMarkdownEditor.dropzoneSubmitReload();
     } catch (error) {
-      showErrorToast(`Failed to save the content: ${error}`);
+      showErrorToast(`Failed to save the content: ${errorMessage(error)}`);
       console.error(error);
     } finally {
       renderContent.classList.remove('is-loading');