Add custom accent color with system color picker

Add a Custom swatch to the appearance settings accent color grid,
backed by a hex value stored in settings (synced via iCloud like the
rest). iOS uses the native ColorPicker wheel; macOS uses a circular
swatch that opens NSColorPanel, since the SwiftUI color well looks out
of place among the circles. All accent consumers now read the resolved
color through SettingsManager.resolvedAccentColor.

The indigo preset is retired from the grid but kept in the enum, so
users who selected it keep their color until they pick another one.

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

YatteeTests/SettingsTests.swift

@@ -65,6 +65,20 @@ struct AccentColorTests {
             #expect(color == decoded)
         }
     }
+
+    @Test("Presets exclude custom and retired indigo")
+    func presetsExcludeHiddenCases() {
+        #expect(!AccentColor.presets.contains(.custom))
+        #expect(!AccentColor.presets.contains(.indigo))
+        #expect(AccentColor.presets.count == AccentColor.allCases.count - 2)
+    }
+
+    @Test("Retired indigo still decodes and resolves")
+    func retiredIndigoStillWorks() throws {
+        let decoded = try JSONDecoder().decode(AccentColor.self, from: Data("\"indigo\"".utf8))
+        #expect(decoded == .indigo)
+        #expect(AccentColor(rawValue: "indigo") == .indigo)
+    }
 }
 
 // MARK: - VideoQuality Tests