Add cross-platform app icons and update Info.plist

- Generated new terminal-themed icon (>_ prompt on dark charcoal)
- macOS: .icns with all retina sizes (16-1024), via iconutil
- Windows: multi-resolution .ico (16-256)
- Linux: individual PNGs (16-512)
- Info.plist: set bundle ID to com.nexusterminal.app, added
  LSApplicationCategoryType (developer-tools), disabled automatic
  and sudden termination
- wails.json: added Windows icon path

wails.json

@@ -15,5 +15,8 @@
     "productVersion": "0.1.0",
     "copyright": "Copyright © 2024 Nexus Terminal Contributors",
     "comments": "A premium desktop SSH, Telnet, and RAW TCP client"
+  },
+  "windows": {
+    "icon": "build/windows/icon.ico"
   }
 }