security(tauri): remove unused shell plugin and capability

- Remove tauri-plugin-shell from Cargo.toml
- Drop tauri_plugin_shell::init() from src-tauri/src/lib.rs
- Delete "shell:allow-open" from src-tauri/capabilities/default.json
- No runtime behavior change; opener plugin still handles links/paths
- Motivation: reduce permissions surface and slightly shrink bundle

src-tauri/src/lib.rs

@@ -9,7 +9,6 @@ use tauri::Manager;
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     tauri::Builder::default()
-        .plugin(tauri_plugin_shell::init())
         .plugin(tauri_plugin_opener::init())
         .setup(|app| {
             #[cfg(target_os = "macos")]