feat(rust): update macos activation policy

[qxtaiba]

Current behavior

The macOS desktop application shows up in both the top right menu and the macOS dock.

Proposed changes

The issue that this PR fixes is described in #5365

This PR implements some platform-specific code that sets the activation policy for macOS. The activation policy is now explicitly configured to display the application only in the macOS menu bar and not in the dock.

Checks

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[qxtaiba]

If you anticipate introducing more specific platform behaviour in the future, then we could also consider refactoring this PR to help improve code organization and make it easier to manage platform-specific functionalities.

main.rs:

#[cfg(target_os = "macos")]
mod macos;
#[cfg(not(target_os = "macos"))]
mod others;

#[cfg(target_os = "macos")]
use macos as platform;
#[cfg(not(target_os = "macos"))]
use others as platform;

lib.rs:

#[cfg_attr(mobile, tauri::mobile_entry_point)]
pub fn run() {
    let app_state = AppState::new();

    // For now the application only consists in a system tray with several menu items
    let mut app = tauri::Builder::default()
        .plugin(configure_tauri_plugin_log())
        .setup(move |app| setup_app(app))
        .system_tray(make_system_tray(&app_state.options()))
        .on_system_tray_event(process_system_tray_event)
        .manage(app_state)
        .build(tauri::generate_context!())
        .expect("Error while building the Ockam application");

    platform::set_platform_activation_policy(&mut app);

    app.run(process_application_event);
}

macos.rs:

use tauri::AppBuilder;

pub fn set_platform_activation_policy(app: &mut AppBuilder<()>) {
    app.set_activation_policy(tauri::ActivationPolicy::Accessory);
}

others.rs:

use tauri::AppBuilder;

pub fn set_platform_activation_policy(_: &mut AppBuilder<()>) {
    // On non-macOS platforms, do nothing
}

[etorreborre]

Thanks @qxtaiba! Let's apply your refactor suggestion if we end-up adding more platform specific behaviors.

[etorreborre]

@qxtaiba you need to rename your commit and prefix it with feat(rust): to make the build pass.