Update Match3D/image-generation docs & code

Adds/updates documentation, assets and implementation for Match3D and puzzle image generation workflows. Key changes: decision logs and pitfalls updated to prefer VectorEngine Gemini for Match3D material sheets and to require edits (multipart) for 1:1 container reference images; guidance added for when to use APIMart vs VectorEngine. .env.example clarified APIMart/Responses config. Many new public assets and PPT visuals added. Code changes across frontend and backend: updated shared contracts, server-rs match3d/puzzle/image-generation handlers, VectorEngine/OpenAI image generation clients, and multiple React components/tests to handle UI/background/container image signing, edits workflow, and puzzle UI background resolution. Added src/services/puzzle-runtime/puzzleUiBackgroundSource.ts and related test updates. Includes notes about multipart HTTP/1.1 requirement and test/verification commands in docs.

src/components/ResolvedAssetImage.tsx

@@ -1,4 +1,4 @@
-import type { ImgHTMLAttributes } from 'react';
+import { type ImgHTMLAttributes, useEffect, useState } from 'react';
 
 import { useResolvedAssetReadUrl } from '../hooks/useResolvedAssetReadUrl';
 
@@ -16,16 +16,42 @@ export function ResolvedAssetImage({
   fallbackSrc,
   alt,
   refreshKey,
+  onError,
   ...rest
 }: ResolvedAssetImageProps) {
   const { resolvedUrl } = useResolvedAssetReadUrl(src, {
     refreshKey,
   });
-  const finalSrc = resolvedUrl || fallbackSrc?.trim() || '';
+  const normalizedFallbackSrc = fallbackSrc?.trim() ?? '';
+  const [useFallbackSrc, setUseFallbackSrc] = useState(false);
+  const finalSrc =
+    useFallbackSrc && normalizedFallbackSrc
+      ? normalizedFallbackSrc
+      : resolvedUrl || normalizedFallbackSrc;
+
+  useEffect(() => {
+    setUseFallbackSrc(false);
+  }, [normalizedFallbackSrc, resolvedUrl]);
 
   if (!finalSrc) {
     return null;
   }
 
-  return <img {...rest} src={finalSrc} alt={alt} />;
+  return (
+    <img
+      {...rest}
+      src={finalSrc}
+      alt={alt}
+      onError={(event) => {
+        if (
+          normalizedFallbackSrc &&
+          !useFallbackSrc &&
+          finalSrc !== normalizedFallbackSrc
+        ) {
+          setUseFallbackSrc(true);
+        }
+        onError?.(event);
+      }}
+    />
+  );
 }