feat: frameless title bar for Windows/Linux with native overlay controls

Use titleBarStyle: 'hidden' + titleBarOverlay on Win/Linux to remove the
ugly native title bar and menu bar while keeping OS-rendered min/max/close
buttons. Hidden Edit menu preserves keyboard shortcuts (Ctrl+C/V/X/Z/A).
Title bar drag region and separator line rendered via web frontend, with
colors matching the Aether Drift design system.

Also adds electron-builder metadata (description, author, homepage,
artifactName) and multi-size icons for cross-platform builds.

packages/desktop/electron-builder.yml

@@ -1,5 +1,6 @@
 appId: com.backspace.desktop
 productName: Backspace
+artifactName: "${productName}-${version}-${arch}.${ext}"
 directories:
   output: dist-electron
 files: