fix: use || instead of ?? for error message fallback

The ?? operator only falls back on null/undefined, so an Error with
an empty message (new Error("")) would produce an empty string and
render a blank error block. Switch to || so empty strings also fall
through to String(error), which produces a non-empty representation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: olaservo

client/src/App.tsx

@@ -955,7 +955,7 @@ const App = () => {
       }));
     } catch (error) {
       console.error(`[App] Failed to read resource ${uri}:`, error);
-      const errorString = (error as Error).message ?? String(error);
+      const errorString = (error as Error).message || String(error);
       setResourceErrorMap((prev) => ({
         ...prev,
         [uri]: errorString,