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/types.ts

@@ -60,6 +60,16 @@ export interface Attachment {
   path: string;
   /** Sent to Ollama next to the base64 payload. */
   mime: string;
+  /**
+   * Pixel dimensions, read from the file's header.
+   *
+   * Kept because the transcript has to lay the preview out long after the file
+   * was read, and Vicinae sizes an image's container to the request rather than
+   * to the image, which leaves dead space unless the request has the right
+   * aspect ratio. Absent for a format whose header we do not parse.
+   */
+  width?: number;
+  height?: number;
 }
 
 /**
@@ -74,7 +84,6 @@ export interface PendingAttachment extends Attachment {
   /** Base64 of the image, with no `data:` URL prefix. Ollama wants it bare. */
   base64: string;
 }
-
 /** What a tool actually returned during a turn, kept for display and debugging. */
 export interface ToolCallRecord {
   name: string;