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/file-view.ts

@@ -2,6 +2,7 @@ import type {InplaceRenderPlugin} from '../render/plugin.ts';
 import {newInplacePluginPdfViewer} from '../render/plugins/inplace-pdf-viewer.ts';
 import {registerGlobalInitFunc} from '../modules/observer.ts';
 import {createElementFromHTML} from '../utils/dom.ts';
+import {errorMessage} from '../modules/errors.ts';
 import {html} from '../utils/html.ts';
 import {basename} from '../utils.ts';
 
@@ -30,7 +31,7 @@ async function renderRawFileToContainer(container: HTMLElement, rawFileLink: str
       rendered = true;
     }
   } catch (e) {
-    errorMsg = `${e}`;
+    errorMsg = errorMessage(e);
   } finally {
     container.classList.remove('is-loading');
   }