feat(update): show release notes + version-card layout in update notice

The update notice dialog now mirrors what GitHub publishes: side-by-side
Installed → Latest version cards (the new one accent-tinted to draw the
eye) and a scrollable release-notes panel under them. Light markdown
rendering covers `##` headings, bullet lists, and inline bold/code
markers — enough for the GitHub release body format we ship.

- update_check::check_for_update() returns UpdateInfo { version, body };
  body is parsed from the releases API JSON with a tiny custom
  string-field extractor (\n / \" / \u handling) so we keep the
  serde_json-free dependency footprint.
- Window grows from 480x240 to 560x460 and becomes resizable to make
  room for the notes scroller.
- Layout tightens: header bottom padding drops 14 → 6, notes heading
  10, and the notes block is wrapped in a Fill-height container so it
  claims every leftover pixel — no more dead space between heading and
  footer.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/view.rs

@@ -195,7 +195,8 @@ impl H7CAD {
                 .update_notice_version
                 .as_deref()
                 .unwrap_or("?");
-            return crate::ui::update_notice::view_window(latest);
+            let body = self.update_notice_body.as_deref().unwrap_or("");
+            return crate::ui::update_notice::view_window(latest, body);
         }
         if Some(window_id) == self.unsaved_dialog_window {
             let tab_name = match &self.pending_close {