Implement drag and drop attachments in chat

[Simplifine-gamedev]

This PR implements drag-and-drop functionality for attachments into the AI chatbox, addressing part of issue ORC-51.

The existing drag-and-drop handling logic (can_drop_data() and drop_data()) was already robust. However, the UI controls where users would naturally drop files (the input field and its surrounding panel) were not configured to receive drag events.

This change enables set_drag_forwarding on the bottom_panel (which contains the attachment area and input field) and directly on the input_field itself. This ensures that files dragged from the OS, Godot's FileSystem dock, or scene nodes can now be correctly dropped into the chat input area to be attached.

This significantly improves the attachment flow and user experience as described in ORC-51.

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Linear Issue: ORC-51

 

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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