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-sidebar-combolist.ts

@@ -2,6 +2,7 @@ import {fomanticQuery} from '../modules/fomantic/base.ts';
 import {GET, POST} from '../modules/fetch.ts';
 import {showErrorToast} from '../modules/toast.ts';
 import {addDelegatedEventListener, queryElemChildren, queryElems, toggleElem} from '../utils/dom.ts';
+import {errorMessage} from '../modules/errors.ts';
 import {parseDom} from '../utils.ts';
 
 export function syncIssueMainContentTimelineItems(oldMainContent: Element, newMainContent: Element) {
@@ -42,7 +43,7 @@ export class IssueSidebarComboList {
   elDropdown: HTMLElement;
   elList: HTMLElement | null;
   elComboValue: HTMLInputElement;
-  initialValues: string[];
+  initialValues: string[] = [];
   container: HTMLElement;
 
   elIssueMainContent: HTMLElement;
@@ -129,7 +130,7 @@ export class IssueSidebarComboList {
       await this.reloadPagePartially();
     } catch (e) {
       console.error('Failed to update to backend', e);
-      showErrorToast(`Failed to update to backend: ${e}`);
+      showErrorToast(`Failed to update to backend: ${errorMessage(e)}`);
     } finally {
       this.elIssueSidebar.classList.remove('is-loading');
     }