fix: add NixOS compatibility for electron startup (#23)

- Add start.sh script that auto-detects NixOS and configures electron path
- Script dynamically finds the correct electron version from nix store
- Keeps original electron-forge start available as start:forge
- Fixes electron launch issues on NixOS systems

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

package.json

@@ -5,7 +5,8 @@
   "main": ".vite/build/main.js",
   "repository": "stoatchat/desktop",
   "scripts": {
-    "start": "electron-forge start",
+    "start": "./start.sh",
+    "start:forge": "electron-forge start",
     "package": "electron-forge package",
     "make": "electron-forge make",
     "publish": "electron-forge publish",