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-diff.ts

@@ -6,6 +6,7 @@ import {initViewedCheckboxListenerFor, initExpandAndCollapseFilesButton} from '.
 import {initImageDiff} from './imagediff.ts';
 import {showErrorToast} from '../modules/toast.ts';
 import {queryElemSiblings, hideElem, showElem, animateOnce, addDelegatedEventListener, createElementFromHTML, queryElems} from '../utils/dom.ts';
+import {errorMessage} from '../modules/errors.ts';
 import {POST, GET} from '../modules/fetch.ts';
 import {createTippy} from '../modules/tippy.ts';
 import {invertFileFolding} from './file-fold.ts';
@@ -85,7 +86,7 @@ function initRepoDiffConversationForm() {
       }
     } catch (error) {
       console.error('Error:', error);
-      showErrorToast(`Submit form failed: ${error}`);
+      showErrorToast(`Submit form failed: ${errorMessage(error)}`);
     } finally {
       form?.classList.remove('is-loading');
     }