Fix confirm-remove dialog close handling

[multiplex55]

Motivation

• Prevent a Rust borrow-checker conflict caused by mutating open inside the egui::Window::show closure which borrows open mutably twice.
• Ensure dialog close requests triggered from UI buttons are handled without violating borrow rules.
• Remove unnecessary mutability on the slot removal tracking variable to simplify the code.

Description

• Added a local should_close flag and set open = false after the show call to defer closing outside the closure and avoid double mutable borrows.
• Replaced in-closure assignments to open with assignments to should_close.
• Removed the redundant mut on the removed variable by changing let mut removed = ... to let removed = ....
• Changes are localized to src/gui/dashboard_editor_dialog.rs around the confirm-remove dialog handling.

Testing

• No automated tests or builds were run as part of this change.
• No test failures reported (no tests executed).

---

Codex Task