feat(qbz-slint): header redesign, Lucide icons, scrollbar fixes

- Replace every Unicode glyph placeholder with Lucide icons via QbzIcon
  (player transport, carousel nav, album-card overlay, album-page
  actions, gear, search, checkbox, sidebar).
- Header: the search field is centered to the window width and gets a
  search icon; the header and player bar now use the elevated surface,
  so all chrome (header, sidebar, player bar) shares one color over the
  darker content area.
- The sidebar user card moves to the header as a user element (name +
  QBZ logo) that opens a dropdown — Settings, Keyboard Shortcuts,
  Documentation, Report an Issue, About, plus Log Out and Close. Log Out
  clears the session and returns to login; Close quits the app.
- Content ScrollViews disable the horizontal scrollbar — no view scrolls
  horizontally, so the phantom reserved lane is gone.

cc

crates/qbz-slint/ui/state.slint

@@ -51,3 +51,8 @@ export global AlbumState {
 export global NavState {
     in-out property <bool> showing-album: false;
 }
+
+// Authenticated user, for the header user element.
+export global SessionState {
+    in property <string> user-name: "";
+}