feat(agents): show images in sent messages (#439)

Images sent to an agent were dropped from the conversation: the user message kept only text and file chips. User entries now record their images and show them as thumbnails that open full size; composer thumbnails open the same preview. Editing a sent message keeps its images (removable) and rerunning from it sends them again.

src/features/agents/ImagePreview.module.css

@@ -0,0 +1,7 @@
+.full {
+  display: block;
+  max-width: 100%;
+  max-height: 70vh;
+  margin: 0 auto;
+  object-fit: contain;
+}