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/utils/match.ts

@@ -2,6 +2,7 @@ import emojis from '../../../assets/emoji.json' with {type: 'json'};
 import {GET} from '../modules/fetch.ts';
 import {showErrorToast} from '../modules/toast.ts';
 import {parseIssuePageInfo} from '../utils.ts';
+import {errorMessage} from '../modules/errors.ts';
 import type {Issue, Mention} from '../types.ts';
 
 const maxMatches = 6;
@@ -47,7 +48,7 @@ export function fetchMentions(mentionsUrl: string): Promise<Mention[]> {
       if (!res.ok) throw new Error(res.statusText);
       return await res.json() as Mention[];
     } catch (e) {
-      showErrorToast(`Failed to load mentions: ${e}`);
+      showErrorToast(`Failed to load mentions: ${errorMessage(e)}`);
       return [];
     }
   })();