Add separate light/dark mode accent colors

Adds an option in Appearance settings to use different accent colors
in light and dark mode, with a toggle to keep a single shared color
(the default, preserving existing behavior).

- New synced settings keys: accentColorDark, customAccentColorDark,
  useSeparateDarkAccentColor; dark values fall back to the light
  selection until explicitly set, so no migration is needed
- resolvedAccentColor returns a dynamic platform color (UIColor trait
  provider / NSColor appearance provider) when the toggle is on, so
  the root tint and all direct readers adapt to light/dark
  automatically without consumer changes
- Accent Color section shows two stacked preset+custom grids with
  Light/Dark headers when enabled, extracted into AccentColorGrid;
  CustomAccentColorButton now takes bindings so the macOS color panel
  edits whichever target was last opened
- New DarkAccentColorTests covering sync contract, default-off state,
  fallbacks, and toggle-off resolution

Claude-Session: https://claude.ai/code/session_0154KH8RAVAvm6iVanhmoW8o

Yattee/Core/Settings/SettingsKey.swift

@@ -14,6 +14,9 @@ enum SettingsKey: String, CaseIterable {
     case theme
     case accentColor
     case customAccentColor
+    case accentColorDark
+    case customAccentColorDark
+    case useSeparateDarkAccentColor
     case showWatchedCheckmark
 
     // Playback

Yattee/Core/Settings/SettingsManager+General.swift

@@ -7,9 +7,9 @@
 
 import Foundation
 import SwiftUI
-#if os(iOS)
+#if canImport(UIKit)
 import UIKit
-#elseif os(macOS)
+#elseif canImport(AppKit)
 import AppKit
 #endif
 
@@ -50,11 +50,78 @@ extension SettingsManager {
         }
     }
 
-    /// The effective accent color, resolving `.custom` to the user-picked value.
-    var resolvedAccentColor: Color {
+    /// Whether the user picked a separate accent color for dark mode.
+    var useSeparateDarkAccentColor: Bool {
+        get {
+            if let cached = _useSeparateDarkAccentColor { return cached }
+            return bool(for: .useSeparateDarkAccentColor, default: false)
+        }
+        set {
+            _useSeparateDarkAccentColor = newValue
+            set(newValue, for: .useSeparateDarkAccentColor)
+        }
+    }
+
+    /// Accent color choice for dark mode. Falls back to the light selection until explicitly set.
+    var accentColorDark: AccentColor {
+        get {
+            if let cached = _accentColorDark { return cached }
+            if let raw = string(for: .accentColorDark), let value = AccentColor(rawValue: raw) {
+                return value
+            }
+            return accentColor
+        }
+        set {
+            _accentColorDark = newValue
+            set(newValue.rawValue, for: .accentColorDark)
+        }
+    }
+
+    /// Custom accent color for dark mode. Falls back to the light custom color until explicitly set.
+    var customAccentColorDark: Color {
+        get {
+            if let hex = _customAccentColorDark ?? string(for: .customAccentColorDark),
+               let color = Color(hex: hex) {
+                return color
+            }
+            return customAccentColor
+        }
+        set {
+            let hex = newValue.toHexString()
+            _customAccentColorDark = hex
+            set(hex, for: .customAccentColorDark)
+        }
+    }
+
+    /// Light (or shared) accent color, resolving `.custom` to the user-picked value.
+    private var resolvedLightAccentColor: Color {
         accentColor == .custom ? customAccentColor : accentColor.color
     }
 
+    /// Dark accent color, resolving `.custom` to the user-picked value.
+    private var resolvedDarkAccentColor: Color {
+        accentColorDark == .custom ? customAccentColorDark : accentColorDark.color
+    }
+
+    /// The effective accent color. When a separate dark color is enabled this is a
+    /// dynamic platform color that resolves per trait environment, so all consumers
+    /// (root tint and direct readers) adapt to light/dark automatically.
+    var resolvedAccentColor: Color {
+        let light = resolvedLightAccentColor
+        guard useSeparateDarkAccentColor else { return light }
+        let dark = resolvedDarkAccentColor
+        #if os(macOS)
+        return Color(nsColor: NSColor(name: nil, dynamicProvider: { appearance in
+            let isDark = appearance.bestMatch(from: [.aqua, .darkAqua]) == .darkAqua
+            return NSColor(isDark ? dark : light)
+        }))
+        #else
+        return Color(uiColor: UIColor { traits in
+            UIColor(traits.userInterfaceStyle == .dark ? dark : light)
+        })
+        #endif
+    }
+
     // MARK: - App Icon Settings
 
     var appIcon: AppIcon {

Yattee/Core/SettingsManager.swift

@@ -29,6 +29,9 @@ final class SettingsManager {
     var _theme: AppTheme?
     var _accentColor: AccentColor?
     var _customAccentColor: String?
+    var _accentColorDark: AccentColor?
+    var _customAccentColorDark: String?
+    var _useSeparateDarkAccentColor: Bool?
     var _showWatchedCheckmark: Bool?
 
     // Playback
@@ -419,6 +422,9 @@ final class SettingsManager {
         _theme = nil
         _accentColor = nil
         _customAccentColor = nil
+        _accentColorDark = nil
+        _customAccentColorDark = nil
+        _useSeparateDarkAccentColor = nil
         _showWatchedCheckmark = nil
         _preferredQuality = nil
         _cellularQuality = nil