feat(app): show loading and error states for assistant images

Render an activity indicator while image dimensions resolve and an
"Image unavailable" fallback if loading fails, replacing the previous
behavior where the surface stayed blank. Reserves a fixed minimum
height in both states so message layout doesn't shift.

packages/app/src/components/message.tsx

@@ -81,8 +81,10 @@ import { getMarkdownListMarker } from "@/utils/markdown-list";
 import { openExternalUrl } from "@/utils/open-external-url";
 import { splitMarkdownBlocks } from "@/utils/split-markdown-blocks";
 import {
+  getAssistantImageLoadStateFromMetadata,
   getAssistantImageMetadata,
   setAssistantImageMetadata,
+  type AssistantImageLoadState,
 } from "@/utils/assistant-image-metadata";
 import { setAssistantMarkdownBlockHeight } from "@/utils/assistant-message-height-estimate";
 import { resolveAssistantImageSource } from "@/utils/assistant-image-source";
@@ -318,7 +320,7 @@ const userMessageStylesheet = StyleSheet.create((theme) => ({
     flexDirection: "row",
     justifyContent: "flex-end",
     paddingHorizontal: theme.spacing[2],
-    userSelect: isWeb ? "text" : "auto",
+    ...(isWeb ? { userSelect: "text" as const } : {}),
   },
   content: {
     alignItems: "flex-end",
@@ -493,7 +495,7 @@ export const assistantMessageStylesheet = StyleSheet.create((theme) => ({
   container: {
     paddingHorizontal: theme.spacing[2],
     paddingVertical: theme.spacing[3],
-    userSelect: isWeb ? "text" : "auto",
+    ...(isWeb ? { userSelect: "text" as const } : {}),
   },
   containerCompactTop: {
     paddingTop: 0,
@@ -567,17 +569,17 @@ const AssistantMarkdownResolvedImage = memo(function AssistantMarkdownResolvedIm
     () => getAssistantImageMetadata({ source, workspaceRoot, serverId }),
     [serverId, source, workspaceRoot],
   );
-  const [aspectRatio, setAspectRatio] = useState<number | null>(
-    cachedMetadata?.aspectRatio ?? null,
+  const [loadState, setLoadState] = useState<AssistantImageLoadState>(() =>
+    getAssistantImageLoadStateFromMetadata(cachedMetadata),
   );
 
   useEffect(() => {
     if (cachedMetadata) {
-      setAspectRatio(cachedMetadata.aspectRatio);
+      setLoadState(getAssistantImageLoadStateFromMetadata(cachedMetadata));
       return;
     }
 
-    setAspectRatio(null);
+    setLoadState({ status: "loading" });
     let cancelled = false;
 
     Image.getSize(
@@ -591,14 +593,17 @@ const AssistantMarkdownResolvedImage = memo(function AssistantMarkdownResolvedIm
             { source, workspaceRoot, serverId },
             { width, height },
           );
-          setAspectRatio(metadata?.aspectRatio ?? width / height);
+          setLoadState({
+            status: "ready",
+            aspectRatio: metadata?.aspectRatio ?? width / height,
+          });
         }
       },
       () => {
         if (cancelled) {
           return;
         }
-        setAspectRatio(null);
+        setLoadState({ status: "error" });
       },
     );
 
@@ -607,19 +612,41 @@ const AssistantMarkdownResolvedImage = memo(function AssistantMarkdownResolvedIm
     };
   }, [cachedMetadata, serverId, source, uri, workspaceRoot]);
 
+  const handleImageError = useCallback(() => {
+    setLoadState({ status: "error" });
+  }, []);
   const surfaceStyle = useMemo<StyleProp<ViewStyle>>(
     () => [
       assistantMessageStylesheet.imageSurface,
-      aspectRatio ? { aspectRatio } : { minHeight: ASSISTANT_IMAGE_MIN_HEIGHT },
+      loadState.status === "ready"
+        ? { aspectRatio: loadState.aspectRatio }
+        : { height: ASSISTANT_IMAGE_MIN_HEIGHT },
     ],
-    [aspectRatio],
+    [loadState],
   );
   const frameStyle = useMemo<StyleProp<ViewStyle>>(
     () => [assistantMessageStylesheet.imageFrame, containerStyle],
     [containerStyle],
   );
+  const stateSurfaceStyle = useMemo<StyleProp<ViewStyle>>(
+    () => [surfaceStyle, assistantMessageStylesheet.imageState],
+    [surfaceStyle],
+  );
   const imageSource = useMemo(() => ({ uri }), [uri]);
 
+  if (loadState.status !== "ready") {
+    return (
+      <View style={frameStyle}>
+        <View style={stateSurfaceStyle}>
+          {loadState.status === "loading" ? <ActivityIndicator size="small" /> : null}
+          {loadState.status === "error" ? (
+            <Text style={assistantMessageStylesheet.imageErrorText}>Image unavailable</Text>
+          ) : null}
+        </View>
+      </View>
+    );
+  }
+
   return (
     <View style={frameStyle}>
       <View style={surfaceStyle}>
@@ -628,6 +655,7 @@ const AssistantMarkdownResolvedImage = memo(function AssistantMarkdownResolvedIm
           style={assistantMessageStylesheet.image}
           resizeMode="contain"
           accessibilityLabel={alt}
+          onError={handleImageError}
         />
       </View>
     </View>
@@ -728,6 +756,7 @@ function AssistantMarkdownImage({
     () => [
       assistantMessageStylesheet.imageFrame,
       containerStyle,
+      { height: ASSISTANT_IMAGE_MIN_HEIGHT },
       assistantMessageStylesheet.imageState,
     ],
     [containerStyle],