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/utils/assistant-image-metadata.ts

@@ -8,6 +8,11 @@ export interface AssistantImageMetadata {
   aspectRatio: number;
 }
 
+export type AssistantImageLoadState =
+  | { status: "loading" }
+  | { status: "ready"; aspectRatio: number }
+  | { status: "error" };
+
 const assistantImageMetadataCache = new Map<string, AssistantImageMetadata>();
 const assistantImageParseCache = new Map<string, { sources: string[]; hasNonImageText: boolean }>();
 const ASSISTANT_IMAGE_METADATA_CACHE_LIMIT = 500;
@@ -126,6 +131,15 @@ export function getAssistantImageMetadata(input: {
   return null;
 }
 
+export function getAssistantImageLoadStateFromMetadata(
+  metadata: AssistantImageMetadata | null,
+): AssistantImageLoadState {
+  if (!metadata) {
+    return { status: "loading" };
+  }
+  return { status: "ready", aspectRatio: metadata.aspectRatio };
+}
+
 export function setAssistantImageMetadata(
   input: {
     source: string;