Linux: Create agent dialog blocks all input and makes the window transparent

## Description

The **Create agent** dialog opens, but none of its fields accept mouse or keyboard input. After a while, the Buzz webview becomes completely transparent and only the native minimize, maximize, and close buttons remain visible.

Editing an existing agent works correctly, even though it uses a very similar dialog. This appears specific to the create path.

## Steps to reproduce

1. Open Buzz Desktop.
2. Go to **Agents**.
3. Click the **+** card to create an agent.
4. Try clicking or typing in **Agent name** or **Agent instructions**.
5. Leave the dialog open for a short while.

## Actual behavior

- The create dialog appears dimmed.
- Inputs cannot be clicked or typed into.
- The rest of Buzz is blocked by the modal.
- Eventually the window content can become fully transparent.
- Only the native title-bar controls remain visible.
- Editing an existing agent continues to work.

## Expected behavior

The Create agent fields should accept pointer and keyboard input, and the window should remain rendered.

## Environment

- Buzz Desktop: **0.5.5**
- OS: **Ubuntu 24.04.4 LTS**
- Desktop: **GNOME on Wayland**
- GPU: **Intel Alder Lake integrated graphics**
- WebKitGTK: **2.52.3**
- Orca/AT-SPI accessibility: **enabled**

## Troubleshooting attempted

The behavior persisted when testing these rendering workarounds individually or together:

- `WEBKIT_DISABLE_DMABUF_RENDERER=1`
- `WEBKIT_DISABLE_COMPOSITING_MODE=1`
- `WEBKIT_SKIA_ENABLE_CPU_RENDERING=1`
- X11/XWayland backend

An earlier general WebKit rendering crash was mitigated with `WEBKIT_DISABLE_DMABUF_RENDERER=1`, but the create-only input/modal problem remains.

A screenshot showing the dimmed, unresponsive create dialog is available and can be attached in a follow-up comment.

Start by reproducing the Create agent path in Buzz Desktop on the reported Ubuntu, Wayland, WebKitGTK, and accessibility setup, then compare it with editing an existing agent. Done means the create fields accept pointer and keyboard input and the window remains rendered without becoming transparent.

Written by the indexing model from the issue text.