fix(api): coalesce concurrent isS3Enabled probes

Two related bugs in the previous boolean-only cache:

1. Stampede: `isS3Enabled()` was called from every `DrawingCard` that
   mounts. With several cards mounting in the same render pass, each
   call hit the cache as `null`, fired its own GET /files/config, and
   raced. Now we cache the in-flight Promise so concurrent callers
   coalesce onto a single request.

2. Sticky-false on transient failures: a 401 during the auth-status
   bootstrap or a one-off network blip permanently latched the cache
   to `false`, disabling S3 uploads for the rest of the page lifetime
   even after auth recovered. Only a successful response is cached
   now; failures resolve `false` for that caller but leave the cache
   open for retry.

Change-Id: Ib38aae45e2724057f12b9a8d03a3603381ecef6a
Co-developed-by: Claude <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: OhYee

frontend/src/api/index.ts

@@ -686,22 +686,42 @@ export const updateLibrary = async (items: LibraryItem[]): Promise<LibraryItem[]
 // S3 file upload helpers
 // ---------------------------------------------------------------------------
 
-/** Cache the result of the /files/config probe so we only call it once. */
+/** Cached resolved result, settled successfully at least once. */
 let s3EnabledCache: boolean | null = null;
+/**
+ * In-flight promise so concurrent callers (e.g. many `DrawingCard`s
+ * mounting at once) coalesce onto a single `/files/config` request
+ * instead of stampeding the backend, and so a transient failure on
+ * one caller doesn't poison the cache for everyone.
+ */
+let s3EnabledInFlight: Promise<boolean> | null = null;
 
 /**
  * Returns true when the backend has S3 configured.
- * The result is cached for the lifetime of the page.
+ * The successful result is cached for the lifetime of the page; on a
+ * transient request failure we let the next caller retry rather than
+ * permanently latching to false.
  */
 export const isS3Enabled = async (): Promise<boolean> => {
   if (s3EnabledCache !== null) return s3EnabledCache;
-  try {
-    const response = await api.get<{ s3Enabled: boolean }>("/files/config");
-    s3EnabledCache = response.data.s3Enabled === true;
-  } catch {
-    s3EnabledCache = false;
-  }
-  return s3EnabledCache;
+  if (s3EnabledInFlight) return s3EnabledInFlight;
+
+  s3EnabledInFlight = (async () => {
+    try {
+      const response = await api.get<{ s3Enabled: boolean }>("/files/config");
+      s3EnabledCache = response.data.s3Enabled === true;
+      return s3EnabledCache;
+    } catch {
+      // Don't cache failures — a transient 401 during auth bootstrap or
+      // a network blip would otherwise permanently disable S3 for the
+      // rest of the page lifetime.
+      return false;
+    } finally {
+      s3EnabledInFlight = null;
+    }
+  })();
+
+  return s3EnabledInFlight;
 };
 
 // ---------------------------------------------------------------------------