Text Chat: Add line edit when virtual keyboard appears

### Problem

When using `LineEdit` nodes on mobile, the on-screen keyboard appears. However, if the `LineEdit` is positioned lower on the screen, it becomes hidden behind the virtual keyboard. This makes it difficult for users to see what they are typing.

### Expected Behavior

When a `LineEdit` is focused and the virtual keyboard appears, the text being entered should remain visible to the user.

### Current Behavior

* Focusing on a `LineEdit` opens the virtual keyboard.
* If the `LineEdit` is not at the top of the screen, it gets covered by the keyboard.
* Users cannot see the text they are typing.

### Proposed Solution

Introduce a global `LineEdit` overlay:

* When a `LineEdit` is focused, redirect the input to a single global `LineEdit` drawn above all UI elements.
* This overlay should remain visible even when the virtual keyboard is open, ensuring the user can always see their text input.

### Benefits

* Consistent text input experience across all `LineEdit` nodes.
* Prevents input fields from being hidden behind the keyboard.
* Improves usability on mobile devices.

| Note: there is an insteresting function named "scroll_container.ensure_control_visible(self)"