feat(whats-new): render standalone release-note links as clickable

The What's New / release-notes modal renders the GitHub release body through a
custom block model that stripped inline markup and had no link support, so a
markdown link showed as literal `[text](url)`. Now:
- inline links `[text](url)` collapse to just their text (clean attributions),
- a whole-line link becomes a KIND_LINK block rendered as a clickable accent
  link opened in the browser (a new WhatsNewActions.open-url reusing open::that,
  mirroring about.rs).

Adds a `url` field to WhatsNewBlock plus a small markdown-link parser. The 2.0.2
notes use it for the daemon-wiki link. Built + smoked at release (window ready,
ALSA 192kHz bit-perfect, playback OK).

cc

crates/qbz-ui/ui/state.slint

@@ -5915,6 +5915,7 @@ export struct WhatsNewBlock {
     level: int,
     text: string,
     id: string,
+    url: string,
 }
 
 // One table-of-contents entry (level-0 section headings only).
@@ -5937,4 +5938,6 @@ export global WhatsNewActions {
     // Open the modal AND kick off the GitHub release fetch (Rust).
     callback open();
     callback close();
+    // Open an external URL from a standalone release-notes link (Rust: open::that).
+    callback open-url(string);
 }