feat: inline image display with lightbox

- Add image block type for base64 and URL images
- Parse image/image_url blocks from gateway history
- Render images inline in chat messages (rounded, dark-themed)
- Click-to-zoom lightbox with Escape to close
- Markdown images also use the lightbox component
- Detect base64 images in tool results (e.g. Read tool on image files)
- Support png, jpg, gif, webp formats

src/components/ImageBlock.tsx

@@ -0,0 +1,64 @@
+import { useState, useEffect, useCallback } from 'react';
+import { X } from 'lucide-react';
+
+interface ImageBlockProps {
+  src: string;
+  alt?: string;
+}
+
+function Lightbox({ src, alt, onClose }: ImageBlockProps & { onClose: () => void }) {
+  const handleKey = useCallback((e: KeyboardEvent) => {
+    if (e.key === 'Escape') onClose();
+  }, [onClose]);
+
+  useEffect(() => {
+    document.addEventListener('keydown', handleKey);
+    return () => document.removeEventListener('keydown', handleKey);
+  }, [handleKey]);
+
+  return (
+    <div
+      className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-sm animate-fade-in"
+      onClick={onClose}
+    >
+      <button
+        onClick={onClose}
+        className="absolute top-4 right-4 p-2 rounded-full bg-zinc-800/80 border border-white/10 text-zinc-300 hover:text-white hover:bg-zinc-700/80 transition-colors"
+      >
+        <X size={20} />
+      </button>
+      <img
+        src={src}
+        alt={alt || 'Image'}
+        className="max-w-[90vw] max-h-[90vh] object-contain rounded-xl shadow-2xl"
+        onClick={(e) => e.stopPropagation()}
+      />
+    </div>
+  );
+}
+
+export function ImageBlock({ src, alt }: ImageBlockProps) {
+  const [lightbox, setLightbox] = useState(false);
+
+  return (
+    <>
+      <div className="my-2">
+        <img
+          src={src}
+          alt={alt || 'Image'}
+          className="max-w-full max-h-80 rounded-xl border border-white/8 cursor-pointer hover:brightness-110 transition-all"
+          onClick={() => setLightbox(true)}
+          loading="lazy"
+        />
+      </div>
+      {lightbox && <Lightbox src={src} alt={alt} onClose={() => setLightbox(false)} />}
+    </>
+  );
+}
+
+/** Build a data URL from base64 image data */
+export function buildImageSrc(mediaType: string, data?: string, url?: string): string {
+  if (url) return url;
+  if (data) return `data:${mediaType};base64,${data}`;
+  return '';
+}