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/artist/ArtistPageView.slint

@@ -227,7 +227,14 @@ export component ArtistPageView inherits Rectangle {
 
     background: Theme.surface-main;
 
+    // Main content + optional network sidebar — same flow as Tauri's
+    // .artist-body. The Flickable shrinks when the sidebar opens; the
+    // sidebar itself is a fixed 300px column on the right.
+    HorizontalLayout {
+        spacing: 0px;
+
     Flickable {
+        horizontal-stretch: 1;
         viewport-width: self.width;
         viewport-height: page.preferred-height;
 
@@ -531,4 +538,95 @@ export component ArtistPageView inherits Rectangle {
             }
         }
     }
+
+        // Network sidebar — a 300px column on the right, fills the
+        // available height, separated from the main content by a thin
+        // border. Sections (Origin / Labels / Similar / Relationships /
+        // Discovery) are populated in the following phases; the shell
+        // only renders the header and an empty body.
+        if NetworkSidebarState.open: Rectangle {
+            width: 300px;
+            background: Theme.surface-card;
+
+            // Left border line — 1px Theme.border-subtle, matching the
+            // Tauri `.network-sidebar-column { border-left: 1px solid
+            // var(--bg-tertiary); }`.
+            Rectangle {
+                x: 0px;
+                y: 0px;
+                width: 1px;
+                height: parent.height;
+                background: Theme.border-subtle;
+            }
+
+            VerticalLayout {
+                spacing: 0px;
+
+                // Header — uppercased title on the left, close button
+                // on the right.
+                Rectangle {
+                    height: 44px;
+                    background: Theme.surface-main;
+
+                    HorizontalLayout {
+                        padding-left: 12px;
+                        padding-right: 8px;
+                        spacing: 8px;
+
+                        Text {
+                            text: @tr("ARTIST NETWORK");
+                            color: Theme.text-muted;
+                            font-size: 12px;
+                            font-weight: Typography.semibold;
+                            letter-spacing: 1.2px;
+                            horizontal-stretch: 1;
+                            vertical-alignment: center;
+                        }
+
+                        VerticalLayout {
+                            alignment: center;
+                            Rectangle {
+                                width: 28px;
+                                height: 28px;
+                                border-radius: 6px;
+                                background: close-ta.has-hover
+                                    ? Theme.surface-elevated
+                                    : transparent;
+
+                                QbzIcon {
+                                    source: @image-url("../assets/icons/panel-right-close.svg");
+                                    width: 18px;
+                                    height: 18px;
+                                    x: (parent.width - self.width) / 2;
+                                    y: (parent.height - self.height) / 2;
+                                    tint: close-ta.has-hover
+                                        ? Theme.text-primary
+                                        : Theme.text-muted;
+                                }
+
+                                close-ta := TouchArea {
+                                    mouse-cursor: pointer;
+                                    clicked => {
+                                        NetworkSidebarState.open = false;
+                                        NetworkSidebarActions.toggle();
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+
+                // Header bottom separator.
+                Rectangle {
+                    height: 1px;
+                    background: Theme.border-subtle;
+                }
+
+                // Body — placeholder. Sections land in phases 5-8.
+                Rectangle {
+                    vertical-stretch: 1;
+                }
+            }
+        }
+    }
 }

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>