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/package.json

@@ -2,6 +2,12 @@
   "name": "@backspace/desktop",
   "version": "1.0.0",
   "private": true,
+  "description": "Backspace — a self-hosted, open-source chat platform",
+  "author": {
+    "name": "youruser",
+    "email": "backspace@backspace.chat"
+  },
+  "homepage": "https://github.com/youruser/backspace",
   "main": "dist/main.js",
   "scripts": {
     "build:ts": "tsc",