User experience is too dependent on macOS menu bar extra

### Bug description

Nextcloud Desktop for macOS currently puts several important actions behind the menu bar icon, including restoring the main window and managing or adding accounts.

This becomes a problem when the macOS menu bar has limited space and the Nextcloud icon is not visible or is difficult to find.

In this situation, two common workflows become difficult:

1. Adding a second Nextcloud account after one account is already connected.
2. Restoring the main window after clicking the window close button.

After the main window is closed, Nextcloud continues running in the background. However, clicking the Nextcloud app icon again from Dock, Launchpad, or Finder does not reopen the main window. The user still needs to use the menu bar icon.

If the menu bar icon is not visible, the app may appear to be frozen, impossible to reopen, or impossible to manage.

### Environment

- Application: Nextcloud Desktop Client
- Version: 33.0.6
- Platform: macOS
- OS: macOS 26.5.1, build 25F80
- Update channel: stable
- Bundle ID: com.nextcloud.desktopclient

### Steps to reproduce

1. Start Nextcloud Desktop on macOS.
2. Connect one Nextcloud account.
3. Use a macOS setup with many menu bar icons, so the Nextcloud menu bar icon may be hidden or hard to find.
4. Try to add a second Nextcloud account.
5. Notice that the entry point depends mainly on the menu bar icon.
6. Open the Nextcloud main window and click the window close button.
7. The main window disappears, but the app keeps running in the background.
8. Click the Nextcloud app icon again from Dock, Launchpad, or Finder.
9. The main window does not reappear.

### Expected behavior

- Clicking the Nextcloud app icon from Dock, Launchpad, or Finder should reopen the main window when the app is already running.
- Adding another account should be possible from the main window or account settings.
- The menu bar icon should be a shortcut, not the only entry point for key actions.
- If closing the window only hides it, the user should still have an obvious way to restore it.

### Actual behavior

- The main window can disappear while the app continues running in the background.
- Clicking the app icon again does not restore the main window.
- Restoring the UI depends on the menu bar icon.
- Adding a second account is difficult if the menu bar icon is not visible.
- The app may appear to be frozen or impossible to reopen.

### Observed processes

After the window is closed, Nextcloud-related processes remain running, for example:

```text
/Applications/Nextcloud.app/Contents/MacOS/Nextcloud
/Applications/Nextcloud.app/Contents/PlugIns/FinderSyncExt.appex/Contents/MacOS/FinderSyncExt
```

This background behavior may be expected, but it becomes confusing when the UI cannot be restored from Dock, Launchpad, Finder, or the main app window.

### Workaround

The current workaround is to find the Nextcloud menu bar icon and restore the UI from there.

If the menu bar icon is not visible, the user may need to manually stop the running process:

```bash
pgrep -afil Nextcloud
kill
```

Then Nextcloud can be restarted.

### Suggested improvement

Please consider improving the macOS entry points:

- Reopen the main window when the user clicks the Nextcloud app icon from Dock, Launchpad, or Finder.
- Provide a clear "Add account" entry point in the main window or account settings.
- Do not make the menu bar icon the only way to restore the UI or manage accounts.
- If the close button hides the window instead of quitting the app, consider showing a first-time hint such as: "Nextcloud is still running in the menu bar."
- Consider adding a setting to choose whether closing the window hides the app or quits the app.

### Why this matters

On macOS, menu bar space is limited. If the Nextcloud menu bar icon is hidden or hard to find, users may lose the main way to control the app.

This affects common workflows such as adding a second account and reopening the main window after it was closed.

Reproduce the macOS workflow by closing the main window and activating Nextcloud again from Dock, Launchpad, or Finder, then compare it with restoring the UI through the menu bar icon. Trace the macOS application and window activation entry points; done means the app can restore its main window and expose account management without requiring the menu bar icon.

Written by the indexing model from the issue text.