feat: update capabilities and add clipboard image reading functionality for Linux

- Updated capabilities.json to allow opening shell commands.
- Implemented read_clipboard_image function for Linux using arboard to read images from the clipboard and encode them as base64 PNG.
- Added a stub for read_clipboard_image on non-Linux platforms.
- Registered the new command in the Tauri application.

src-tauri/capabilities/default.json

@@ -5,7 +5,7 @@
   "windows": ["*"],
   "permissions": [
     "core:default",
-    "shell:default",
+    "shell:allow-open",
     "notification:default"
   ]
 }