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/package.json

@@ -5,7 +5,7 @@
   "main": "dist/main.js",
   "scripts": {
     "build:ts": "tsc",
-    "dev": "tsc && electron .",
+    "dev": "mkdir -p build && cp ../../icon.png build/icon.png && node -e \"try{require('child_process').execSync('swift scripts/gen-dock-icon.swift',{stdio:'inherit'})}catch(e){}\" && tsc && electron .",
     "prebuild": "mkdir -p build && cp ../../icon.png build/icon.png",
     "build": "tsc && electron-builder",
     "clean": "rm -rf dist dist-electron"