feat(slint): artist network sidebar shell with header

Restructures ArtistPageView's outer layout into a HorizontalLayout
where the existing Flickable becomes a horizontal-stretch column and a
fixed 300px sidebar opens on the right when NetworkSidebarState.open
is true — mirroring Tauri's .artist-body { content + .network-sidebar-
column } flex row.

Sidebar shell:
  - 300px column with a 1px left border in border-subtle, surface-card
    background.
  - Header row (44px tall, surface-main background, separator line
    below): uppercased "ARTIST NETWORK" label on the left, 28x28 close
    button on the right using the panel-right-close icon (same icon
    Tauri renders via Lucide PanelRightClose).
  - Close button mirrors the header toggle: flips
    NetworkSidebarState.open to false and emits
    NetworkSidebarActions.toggle so the Rust side persists.
  - Body is an empty stretch placeholder; Origin / Labels / Similar /
    Relationships / Discovery sections land in the following phases.

Asset: panel-right-close.svg ported from Lucide (24x24 viewBox,
stroke-width 2, same path as Tauri's lucide-svelte import).

cc

crates/qbz-slint/ui/assets/icons/panel-right-close.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <rect width="18" height="18" x="3" y="3" rx="2" />
+  <path d="M15 3v18" />
+  <path d="m8 9 3 3-3 3" />
+</svg>