feat: persist sidebar width across launches (#26)

Wire an NSSplitView autosave name into NavigationSplitView's underlying
AppKit split view so the sidebar's drag-to-resize position is remembered
in UserDefaults and restored on next launch.

SplitViewAutosave.swift installs an invisible NSViewRepresentable that
walks up the view hierarchy from the sidebar, finds the enclosing
NSSplitView, and assigns autosaveName = "ScarfMainSidebar". AppKit
handles persistence from there — no manual UserDefaults or @AppStorage
plumbing needed.

ContentView also gets navigationSplitViewColumnWidth(min:ideal:max:)
bounds so first-launch (before any autosave exists) lands at a sensible
240pt ideal within a 180–360pt range.

Refs #26

scarf/scarf/ContentView.swift

@@ -14,6 +14,7 @@ struct ContentView: View {
     var body: some View {
         NavigationSplitView {
             SidebarView()
+                .navigationSplitViewColumnWidth(min: 180, ideal: 240, max: 360)
         } detail: {
             detailView
                 .toolbar {

scarf/scarf/Navigation/SidebarView.swift

@@ -59,5 +59,6 @@ struct SidebarView: View {
         }
         .listStyle(.sidebar)
         .navigationTitle("Scarf")
+        .splitViewAutosaveName("ScarfMainSidebar")
     }
 }

scarf/scarf/Navigation/SplitViewAutosave.swift

@@ -0,0 +1,57 @@
+import AppKit
+import SwiftUI
+
+/// Makes the enclosing `NSSplitView` remember its divider positions across
+/// app launches. `NavigationSplitView` is backed by `NSSplitViewController`,
+/// whose split view honours `autosaveName` — AppKit writes the divider
+/// offsets to `UserDefaults` on drag and restores them on the next launch.
+///
+/// Usage: attach `.splitViewAutosaveName("…")` to a child of the split view
+/// (the sidebar is a good choice). The modifier installs an invisible helper
+/// that walks up the view hierarchy on first layout, finds the `NSSplitView`,
+/// and assigns its autosave name. Subsequent launches restore the divider
+/// positions before the window appears.
+///
+/// The name is also used to key the entry in `UserDefaults` (AppKit stores
+/// it as `NSSplitView Subview Frames <name>`), so changing the name resets
+/// the remembered width. Pick a stable string and leave it alone.
+struct SplitViewAutosaveFinder: NSViewRepresentable {
+    let autosaveName: String
+
+    func makeNSView(context: Context) -> NSView {
+        let view = NSView()
+        // Defer the hierarchy walk until after SwiftUI has attached this
+        // view to its host window — at makeNSView time the view has no
+        // superview yet, so we can't find the split view above us.
+        DispatchQueue.main.async { [weak view] in
+            guard let view else { return }
+            SplitViewAutosaveFinder.apply(autosaveName, startingFrom: view)
+        }
+        return view
+    }
+
+    func updateNSView(_ nsView: NSView, context: Context) {}
+
+    private static func apply(_ name: String, startingFrom view: NSView) {
+        var current: NSView? = view
+        while let node = current {
+            if let split = node as? NSSplitView {
+                // Only set once — reassigning clobbers AppKit's restore path.
+                if split.autosaveName != NSSplitView.AutosaveName(name) {
+                    split.autosaveName = NSSplitView.AutosaveName(name)
+                }
+                return
+            }
+            current = node.superview
+        }
+    }
+}
+
+extension View {
+    /// Persist the enclosing `NavigationSplitView` / `NSSplitView` divider
+    /// positions to `UserDefaults` under `autosaveName`. Attach to any child
+    /// of the split view (the sidebar works well).
+    func splitViewAutosaveName(_ autosaveName: String) -> some View {
+        background(SplitViewAutosaveFinder(autosaveName: autosaveName))
+    }
+}