feat: squircle dock icon for macOS dev mode

macOS only applies the squircle mask to packaged .app bundles. For dev
mode, generate a pre-masked icon-dock.png via Swift/AppKit at launch
and set it with app.dock.setIcon(). Silently skipped on non-macOS.

packages/desktop/scripts/gen-dock-icon.swift

@@ -0,0 +1,21 @@
+// Generates a squircle-masked dock icon for macOS dev mode.
+// macOS only applies the squircle mask to packaged .app bundles,
+// so app.dock.setIcon() needs a pre-masked PNG.
+
+import AppKit
+
+let src = NSImage(contentsOfFile: "build/icon.png")!
+let size = src.size
+let out = NSImage(size: size)
+
+out.lockFocus()
+let radius = size.width * 0.2237
+let path = NSBezierPath(roundedRect: NSRect(origin: .zero, size: size), xRadius: radius, yRadius: radius)
+path.addClip()
+src.draw(in: NSRect(origin: .zero, size: size), from: NSRect(origin: .zero, size: size), operation: .sourceOver, fraction: 1.0)
+out.unlockFocus()
+
+let tiff = out.tiffRepresentation!
+let rep = NSBitmapImageRep(data: tiff)!
+let png = rep.representation(using: .png, properties: [:])!
+try! png.write(to: URL(fileURLWithPath: "build/icon-dock.png"))