feat(slint): browser-like back/forward navigation history

Add a navigation history so pages can be walked back and forward.

- nav.rs: a back/forward stack of NavEntry (home, album, artist,
  settings); record() on fresh navigation, go_back/go_forward to walk it
- NavButtons: a [<] [>] pair styled like the Discover tab group; inline
  beside the Discover tabs and in place of the per-page Back button on
  album, artist and settings views
- All navigation routes through Rust (open-album/-artist record entries;
  the gear and header menu use NavState.request-settings) so the history
  stays accurate; back/forward re-apply entries, re-loading entity pages
- AppShell listens for the mouse back/forward buttons
- can-back / can-forward drive the buttons' enabled state

Also in this round:

- Album header description: render a pre-truncated short string so it
  wraps to a couple of lines instead of overflowing the header; the full
  text stays in the Read-more modal
- Header queue button uses an ordered-list icon (the previous one read
  as a playlist)

cc

crates/qbz-slint/ui/state.slint

@@ -64,6 +64,8 @@ export global AlbumState {
     in property <string> quality-detail;
     // Editorial description / review (HTML stripped). May be empty.
     in property <string> description;
+    // Truncated description for the header; full text lives in the modal.
+    in property <string> description-short;
     in property <image> artwork;
     // Artwork-derived tint for the header gradient.
     in property <color> header-color: #181820;
@@ -117,6 +119,13 @@ export global SettingsState {
 
 export global NavState {
     in-out property <ContentView> view: ContentView.home;
+    // Browser-like history flags, driven by the Rust nav module.
+    in property <bool> can-back: false;
+    in property <bool> can-forward: false;
+    // Navigation requests routed to Rust so history is recorded.
+    callback request-back();
+    callback request-forward();
+    callback request-settings();
 }
 
 // Artist detail screen state.