[fix] simple theme: show why the AI summary failed

The error box said only "Error loading the AI summary".  The status of
the failed request was raised and then dropped, so a user could not tell
an LLM server that is unreachable (502) from a plugin that is switched
off (403) or a request that was rejected (400).

The reason is now appended to the message.  On a phone or in a webview,
where dev tools are not an option, it is the only thing the person
looking at the failure has to go on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

client/simple/src/js/plugin/AiSummary.ts

@@ -157,7 +157,9 @@ export default class AiSummary extends Plugin {
         signal: controller.signal
       });
       if (!res.ok) {
-        throw new Error(res.statusText);
+        // the status is shown to the user: it is often the only thing
+        // available to diagnose a failure on a device without dev tools
+        throw new Error(`HTTP ${res.status}`);
       }
 
       if (res.body) {
@@ -195,8 +197,11 @@ export default class AiSummary extends Plugin {
   private static showError(box: HTMLElement, error: unknown): void {
     console.error("Error loading AI summary:", error);
 
+    const message = settings.translations?.error_loading_ai_summary ?? "Error loading the AI summary";
+    const reason = error instanceof Error && error.message ? ` (${error.message})` : "";
+
     const errorElement = Object.assign(document.createElement("div"), {
-      textContent: settings.translations?.error_loading_ai_summary ?? "Error loading the AI summary",
+      textContent: `${message}${reason}`,
       className: "dialog-error"
     });
     errorElement.setAttribute("role", "alert");