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/app.slint

@@ -10,7 +10,7 @@ import { LoginScreen } from "login/LoginScreen.slint";
 import { AppShell } from "shell/AppShell.slint";
 
 // Re-export the state globals so the Rust layer can populate them.
-export { HomeState, AlbumState, NavState } from "state.slint";
+export { HomeState, AlbumState, NavState, SessionState } from "state.slint";
 
 // Which top-level screen is shown. The app starts on `splash` while it
 // restores a saved session, then resolves to `shell` or `login`.
@@ -32,6 +32,8 @@ export component AppWindow inherits Window {
     callback start-offline();
     callback open-tos();
     callback open-album(string);
+    callback logout();
+    callback close-app();
 
     if root.screen == AppScreen.splash: SplashScreen { }
 
@@ -54,5 +56,11 @@ export component AppWindow inherits Window {
         open-album(id) => {
             root.open-album(id);
         }
+        logout => {
+            root.logout();
+        }
+        close-app => {
+            root.close-app();
+        }
     }
 }