Fix: Compatibility with modern NiceGUI and improved theme synchronization

[AlePiccin]

🚀 Overview
This PR addresses compatibility issues with recent versions of NiceGUI and enhances how theme changes are propagated to clients, especially when called from startup hooks or shared contexts.

🛠️ Key Changes
NiceGUI Modernization:

Removed reliance on is_auto_index_client, which is no longer present in recent NiceGUI versions.

Implemented a safer context check using bool(ui.context.slot_stack) to determine if the function is being called within a UI socket context or a global background task (like app.on_startup).

Robust Context Handling:

Fixed the ui.context.client error that occurred when calling use_theme during the application startup phase.

The logic now gracefully handles cases where no active slot/client exists yet.

New Feature: Instant Shared Updates:

Introduced a more dynamic shared logic. When shared=True, the component now iterates through all active Client.instances to inject the necessary JavaScript (_remove_old_theme_js) in real-time.

This ensures that a theme change triggered globally is immediately reflected for all connected users without requiring a page refresh.

📝 Technical Details
JS Injection: Extracted the CSS cleanup logic into a constant _remove_old_theme_js for better maintainability.

Thread Safety: Used list(Client.instances.values()) to prevent "dictionary changed size during iteration" errors when updating multiple clients simultaneously.

Logical Defaults:

If called outside a context: Defaults to shared=True.

If called inside a page: Defaults to shared=False (per-client).

🧪 How to Test
Call use_theme() inside an app.on_startup handler. It should no longer throw a context error.

Open multiple browser tabs. Change the theme with shared=True and verify that all tabs update the Tabulator theme instantly via JS injection.

[CrystalWindSnake]

@AlePiccin Thanks for your PR!