fix: lay the screenshot preview out to the image, not to the request

The preview rendered, but with dead space around it. `MdImage.qml` sizes its
container to the request rather than to the image, so with no hint it is the
full pane width and 200px tall and the image is aspect-fitted inside that. A
wide capture sat in a letterbox: measured against the real captures in the
support directory, a 4:1 crop wasted about 106px of the 200px container and a
6.7:1 crop about 140px, which is what made the transcript look gappy and put
the caption far below the image.

- `readAttachment` now reads the PNG `IHDR` chunk, so `Attachment` carries the
  real dimensions and they are persisted with the turn.
- `previewBox` computes an exact box from those dimensions, scaled to fit
  400x200 with the scale capped at 1 so a small capture is not upscaled, and
  `imageSource` emits it as the `raycast-width`/`raycast-height` hints
  `parseImageSize` looks for. The path is resolved from the URL before the
  query, so the hints never reach the file name.
- The alt text is now "Screenshot". It is drawn centred under the image, so the
  sentence "Screenshot attached" read as a stray line of prose floating in the
  middle of the pane.
- A pending note no longer stacks on top of the empty-transcript placeholder.
  Both said the same thing, one line under the other, once a snip was attached.

A capture with no readable header simply falls back to the old sizing.

Verified: 293 tests, tsc, eslint, prettier, and vici build.

src/lib/components/ChatView.tsx

@@ -190,10 +190,14 @@ export function ChatView({
       pending ? `${renderAttachments([pending])}\n\n${PENDING_NOTE}` : undefined,
     ].filter((note): note is string => Boolean(note));
 
+    // The placeholder is what tells a user that the search bar is the composer.
+    // When a note above already says that, printing both put the same
+    // instruction twice, one line under the other.
+    const empty = notes.length === 0 ? EMPTY_TRANSCRIPT : "";
     const transcript =
-      thread.length === 0 ? EMPTY_TRANSCRIPT : buildThreadMarkdown(thread, transcriptOptions);
+      thread.length === 0 ? empty : buildThreadMarkdown(thread, transcriptOptions);
 
-    return [...notes, transcript].join("\n\n");
+    return [...notes, transcript].filter((part) => part.length > 0).join("\n\n");
   }, [launchNote, snipNote, pending, thread, transcriptOptions]);
 
   const rows: SessionRow[] = useMemo(() => {