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/capabilities/default.json

@@ -7,7 +7,6 @@
   ],
   "permissions": [
     "core:default",
-    "shell:allow-open",
     "opener:default"
   ]
 }