Add macOS .icns icon and update config

Add frontend/assets/icons/GitpubDesktop.icns and update src-tauri/tauri.conf.json to reference the new ICNS path so the macOS build uses the provided icon. Also change package.json name from "gitpub-desktop" to "Gitpub Desktop" (display/casing) for consistency.

package.json

@@ -1,5 +1,5 @@
 {
-  "name": "gitpub-desktop",
+  "name": "Gitpub Desktop",
   "private": true,
   "version": "0.1.0",
   "type": "module",

src-tauri/tauri.conf.json

@@ -31,7 +31,7 @@
       "icons/32x32.png",
       "icons/128x128.png",
       "icons/128x128@2x.png",
-      "icons/icon.icns",
+      "../frontend/assets/icons/GitpubDesktop.icns",
       "../frontend/assets/icons/GitpubDesktop-Icon.ico"
     ]
   }