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/Cargo.toml

@@ -23,7 +23,6 @@ serde = { version = "1.0", features = ["derive"] }
 log = "0.4"
 tauri = { version = "2.8.2", features = [] }
 tauri-plugin-log = "2"
-tauri-plugin-shell = "2"
 tauri-plugin-opener = "2"
 dirs = "5.0"