Closes #14640

This PR ensures that the AI Export button is dynamically disabled when the chat history is empty.
A JavaFX binding was implemented in the AiChatComponent controller to bind the MenuButton.disableProperty() to the empty state of the chat history observable list.
This guarantees consistent behavior across both the Entry Editor AI tab and the "Chat with groups" window, as they share the same underlying component.

Steps to test

1. Initial State

   • Open JabRef and navigate to an entry with an attached PDF.
   • Open the AI chat tab.
   • Verify that the Export button is disabled by default when no chat messages are present.

2. Enabled State

   • Send a message in the AI chat.
   • Once the message appears in the chat history, verify that the Export button becomes enabled immediately.

3. Reset State

   • Click the Clear chat history (trash icon) at the bottom of the chat.
   • Confirm the action.
   • Verify that the Export button returns to a disabled state.

4. Group Chat

   • Right-click a group and select Chat with group.
   • Verify that the Export button behavior matches the entry editor AI tab:
     • Disabled when the chat is empty
     • Enabled after sending a message
     • Disabled again after clearing the chat

Screenshots of the issue being fixed

• AI Tab View (Empty Chat)

• Group Chat Window (Empty Chat)

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.