fix: clean up pending thumbnail map on rejection to prevent stale entries

Move removePending to .finally() so it runs on both resolve and reject,
preventing stale rejected promises from accumulating in the pending map.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useAdaptiveImageSrc.ts

@@ -66,27 +66,32 @@ export function useAdaptiveImageSrc(
     // Check if generation is already in progress
     const existing = getPending(fullSrc);
     if (existing) {
-      existing.then((thumb) => {
-        if (prevSrcRef.current === fullSrc) {
-          setThumbnailSrc(thumb);
-        }
-      });
+      existing
+        .then((thumb) => {
+          if (prevSrcRef.current === fullSrc) {
+            setThumbnailSrc(thumb);
+          }
+        })
+        .catch(() => {});
       return;
     }
 
     // Generate thumbnail
-    const promise = generateThumbnail(fullSrc).then((thumb) => {
-      setThumbnail(fullSrc, thumb);
-      removePending(fullSrc);
-      return thumb;
-    });
+    const promise = generateThumbnail(fullSrc)
+      .then((thumb) => {
+        setThumbnail(fullSrc, thumb);
+        if (prevSrcRef.current === fullSrc) {
+          setThumbnailSrc(thumb);
+        }
+        return thumb;
+      })
+      .finally(() => {
+        removePending(fullSrc);
+      });
     setPending(fullSrc, promise);
 
-    promise.then((thumb) => {
-      if (prevSrcRef.current === fullSrc) {
-        setThumbnailSrc(thumb);
-      }
-    });
+    // Suppress unhandled rejection — fullSrc falls back gracefully
+    promise.catch(() => {});
   }, [fullSrc]);
 
   if (!fullSrc) return null;