Set the navigation and tab bars in the library's type

The screen titles were still system bold sans — `navigationTitle` renders
through UIKit, which no SwiftUI font modifier reaches, so paperSurface()
could tint the bar but never restyle its text. PaperAppearance configures
UINavigationBarAppearance once at launch: serif large and inline titles in
ink, and monospaced tab bar labels.

One trap worth recording: configuring that appearance with
configureWithOpaqueBackground() makes iOS 26 stop laying out the large
title entirely — it vanishes rather than restyling, and it does so even
with the font attributes removed, so it reads like a font problem when it
isn't. Transparent works, and is right here anyway: every screen already
paints the paper ground itself.

Verified in both schemes. Suite passes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JgLHztZGaEHvS3KNeGQmRM

Marks/Views/Library/LibraryKit.swift

@@ -83,11 +83,20 @@ enum Paper {
     }
 
     static func dynamic(light: UInt32, dark: UInt32) -> Color {
-        Color(uiColor: UIColor { traits in
-            UIColor(rgb: traits.userInterfaceStyle == .dark ? dark : light)
-        })
+        Color(uiColor: dynamicUI(light: light, dark: dark))
     }
 
+    static func dynamicUI(light: UInt32, dark: UInt32) -> UIColor {
+        UIColor { traits in
+            UIColor(rgb: traits.userInterfaceStyle == .dark ? dark : light)
+        }
+    }
+
+    /// UIKit equivalents, for the chrome SwiftUI can't reach — chiefly the
+    /// navigation bar, whose title font has no SwiftUI API at all.
+    static let uiSheet = dynamicUI(light: 0xF8F5EF, dark: 0x15130F)
+    static let uiInk = dynamicUI(light: 0x14110C, dark: 0xF1ECE1)
+
     static func luminance(of hex: UInt32) -> Double {
         let r = Double((hex >> 16) & 0xFF) / 255
         let g = Double((hex >> 8) & 0xFF) / 255
@@ -154,6 +163,56 @@ extension View {
     }
 }
 
+// MARK: - UIKit chrome
+
+/// The navigation and tab bars are UIKit underneath, and their fonts have no
+/// SwiftUI equivalent — `navigationTitle` will render in the system bold sans
+/// whatever you do to the view. Both are configured once at launch so screen
+/// titles speak the same serif as the content beneath them.
+enum PaperAppearance {
+    static func apply() {
+        let bar = UINavigationBarAppearance()
+        // Transparent, not opaque: the screens already paint the paper ground
+        // themselves, and an opaque bar config stops iOS 26 laying out the
+        // large title at all.
+        bar.configureWithTransparentBackground()
+        bar.largeTitleTextAttributes = [
+            .font: serif(34, weight: .regular),
+            .foregroundColor: Paper.uiInk,
+        ]
+        bar.titleTextAttributes = [
+            .font: serif(17, weight: .medium),
+            .foregroundColor: Paper.uiInk,
+        ]
+        UINavigationBar.appearance().standardAppearance = bar
+        UINavigationBar.appearance().compactAppearance = bar
+        UINavigationBar.appearance().scrollEdgeAppearance = bar
+
+        let tabs = UITabBarAppearance()
+        tabs.configureWithDefaultBackground()
+        let label: [NSAttributedString.Key: Any] = [
+            .font: UIFont.monospacedSystemFont(ofSize: 10, weight: .medium)
+        ]
+        for item in [tabs.stackedLayoutAppearance,
+                     tabs.inlineLayoutAppearance,
+                     tabs.compactInlineLayoutAppearance] {
+            item.normal.titleTextAttributes = label
+            item.selected.titleTextAttributes = label
+        }
+        UITabBar.appearance().standardAppearance = tabs
+        UITabBar.appearance().scrollEdgeAppearance = tabs
+    }
+
+    /// `withDesign(.serif)` is the only route to the system serif in UIKit, and
+    /// it returns nil if the design is unavailable — fall back rather than
+    /// force-unwrap a font.
+    private static func serif(_ size: CGFloat, weight: UIFont.Weight) -> UIFont {
+        let base = UIFont.systemFont(ofSize: size, weight: weight)
+        guard let descriptor = base.fontDescriptor.withDesign(.serif) else { return base }
+        return UIFont(descriptor: descriptor, size: size)
+    }
+}
+
 // MARK: - Empty state
 
 /// The paper equivalent of `ContentUnavailableView`, which can't be restyled —