fix(desktop): keep generated images in the tool slot, not inline

The image-generate tool showed a placeholder, then the model echoed a
(often different) image inline in its prose — a second, jarring copy in
the wrong place, dimmed as tool scaffolding, with a misplaced download
button.

Now the generated image lives only in the tool slot:
- Strip every embedded image/media link from the assistant prose of a
  message that produced an image (the model frequently restates the
  remote URL while the result holds the local path), preserving the
  agent's words. Applied on hydration, live deltas, and completion.
- One stable frame sized from the aspect_ratio arg up front, so the
  diffusion placeholder and the decoded image share the same box and
  crossfade with no layout shift; the box derives its height from the
  true ratio on load (no letterboxing).
- Exempt generated images from the tool-block dim-until-hover rule.
- Extract a shared useImageDownload hook + ImageLightbox so the tool
  image and markdown images share one implementation.

apps/desktop/src/styles.css

@@ -1180,6 +1180,12 @@ canvas {
   opacity: 1;
 }
 
+/* A generated image is the deliverable, not scaffolding — keep it at full
+   strength instead of dimming it until hover. */
+[data-slot='aui_assistant-message-content'] > [data-slot='tool-block']:has([data-slot='aui_generated-image']) {
+  opacity: 1;
+}
+
 /* Conversation block rhythm. assistant-ui renders each range as a direct child
    of the message content with no per-part wrapper, so adjacency rules cover
    every pairing — first block needs no reset, nested tool rows are untouched.