feat(apple): use the brand purple as the app-wide accent color

The macOS sidebar selection and item icons rendered in the system default
blue because the app had no global accent color — SwiftUI's `.tint` doesn't
reach the AppKit-backed sidebar. Add an AccentColor asset (the exact sRGB of
VniDropColors.brandPurple) and wire ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME
so the accent applies at the OS level everywhere, including the sidebar.

apple/VniDrop/Resources/Assets.xcassets/AccentColor.colorset/Contents.json

@@ -0,0 +1,20 @@
+{
+  "colors" : [
+    {
+      "color" : {
+        "color-space" : "srgb",
+        "components" : {
+          "alpha" : "1.000",
+          "blue" : "0.9685",
+          "green" : "0.3315",
+          "red" : "0.6606"
+        }
+      },
+      "idiom" : "universal"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}

apple/VniDrop/UI/Theme/VniDropColors.swift

@@ -52,7 +52,9 @@ struct VniDropColors {
 }
 
 extension VniDropColors {
-	/// The single brand accent used app-wide as the SwiftUI tint.
+	/// The single brand accent used app-wide as the SwiftUI tint. Mirrored by the
+	/// `AccentColor` asset (the OS-level global accent for the macOS sidebar etc.);
+	/// keep the two in sync.
 	static let brandPurple = Color.hsl(271, 91, 65)
 
 	static let light = VniDropColors(