feat(qbz-slint): app shell frame

M1 progress. Adds the post-login shell: HeaderBar (nav tabs and
search), Sidebar (playlists list and user card), PlayerBar (transport,
now-playing, volume), and AppShell composing them with the content
region as a placeholder.

app.slint gains an AppScreen enum; clicking sign-in or start-offline
navigates login -> shell (no real auth yet — that is M2). New layout
foundation tokens (sidebar width, header/player-bar heights) plus
border and hover semantic colors.

cc

crates/qbz-slint/ui/app.slint

@@ -6,6 +6,12 @@
 
 import { Theme } from "foundation/semantic-colors.slint";
 import { LoginScreen } from "login/LoginScreen.slint";
+import { AppShell } from "shell/AppShell.slint";
+
+// Which top-level screen is shown. During M1 the screen switches on a
+// login-button click without real authentication; M2 ties the switch
+// to a real session.
+export enum AppScreen { login, shell }
 
 export component AppWindow inherits Window {
     title: "QBZ";
@@ -15,13 +21,15 @@ export component AppWindow inherits Window {
     min-height: 600px;
     background: Theme.surface-main;
 
+    in-out property <AppScreen> screen: AppScreen.login;
+
     // Forwarded to the Rust layer, which maps them to typed AppCommands.
     callback sign-in-via-browser();
     callback use-system-browser();
     callback start-offline();
     callback open-tos();
 
-    LoginScreen {
+    if root.screen == AppScreen.login: LoginScreen {
         sign-in-via-browser => {
             root.sign-in-via-browser();
         }
@@ -35,4 +43,6 @@ export component AppWindow inherits Window {
             root.open-tos();
         }
     }
+
+    if root.screen == AppScreen.shell: AppShell { }
 }