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.test.ts

@@ -3,6 +3,7 @@ import {
   clearAssistantImageMetadataCache,
   estimateAssistantMessageHeightFromCache,
   extractAssistantImageSources,
+  getAssistantImageLoadStateFromMetadata,
   getAssistantImageMetadata,
   setAssistantImageMetadata,
 } from "./assistant-image-metadata";
@@ -41,6 +42,23 @@ describe("assistant image metadata", () => {
     });
   });
 
+  it("maps missing metadata to the image loading state", () => {
+    expect(getAssistantImageLoadStateFromMetadata(null)).toEqual({ status: "loading" });
+  });
+
+  it("maps cached metadata to the image ready state", () => {
+    expect(
+      getAssistantImageLoadStateFromMetadata({
+        width: 900,
+        height: 1600,
+        aspectRatio: 9 / 16,
+      }),
+    ).toEqual({
+      status: "ready",
+      aspectRatio: 9 / 16,
+    });
+  });
+
   it("estimates assistant message height from cached image metadata", () => {
     setAssistantImageMetadata(
       {