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/gen/schemas/capabilities.json

@@ -1 +1 @@
-{"default":{"identifier":"default","description":"Default capability for all windows","local":true,"windows":["*"],"permissions":["core:default","shell:default","notification:default"]}}
+{"default":{"identifier":"default","description":"Default capability for all windows","local":true,"windows":["*"],"permissions":["core:default","shell:allow-open","notification:default"]}}