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-common.ts

@@ -1,4 +1,5 @@
 import {queryElems} from '../utils/dom.ts';
+import {errorMessage} from '../modules/errors.ts';
 import {POST} from '../modules/fetch.ts';
 import {showErrorToast} from '../modules/toast.ts';
 import {sleep} from '../utils.ts';
@@ -26,7 +27,7 @@ async function onDownloadArchive(e: Event) {
     window.location.href = el.href; // the archive is ready, start real downloading
   } catch (e) {
     console.error(e);
-    showErrorToast(`Failed to download the archive: ${e}`, {duration: 2500});
+    showErrorToast(`Failed to download the archive: ${errorMessage(e)}`, {duration: 2500});
   } finally {
     targetLoading.classList.remove('is-loading', 'loading-icon-2px');
   }