Fix relative-time error and improve global error handler (#37241)

1. Fixes: #37239
2. Enhance global error message to show stack trace on click

---------

Signed-off-by: silverwind <me@silverwind.io>
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/webcomponents/relative-time.ts

@@ -258,13 +258,13 @@ class RelativeTime extends HTMLElement {
   }
 
   get #lang(): string {
-    const lang = this.closest('[lang]')?.getAttribute('lang');
-    if (lang) {
+    for (const candidate of [this.closest('[lang]')?.getAttribute('lang'), navigator.language]) {
+      if (!candidate) continue;
       try {
-        return new Intl.Locale(lang).toString();
-      } catch { /* invalid locale, fall through */ }
+        return String(new Intl.Locale(candidate));
+      } catch {}
     }
-    return navigator.language ?? 'en';
+    return 'en';
   }
 
   get second(): 'numeric' | '2-digit' | undefined {