fix: import tkinter.messagebox explicitly in ui.py

[rsmxingu]

Problem

ui.py calls tk.messagebox.* in five places (no-file warning, no-image warning, cancel confirmation, error popup, success popup) but only imports tkinter as tk.

tkinter.messagebox is a submodule — it is not loaded by import tkinter. The attribute tk.messagebox only resolves if some other already-imported module happened to import it first (e.g. customtkinter, on some versions). This makes every dialog an AttributeError waiting to happen, depending on the customtkinter/Python version in use.

Affected call sites (ui.py):

• on_optimize_clicked → showwarning (no file / no intro image)
• on_cancel_clicked → askyesno
• update_progress → showerror / showinfo

Change

Import the submodule explicitly and call it directly:

from tkinter import filedialog, messagebox
...
messagebox.showwarning(...)  # instead of tk.messagebox.showwarning(...)

Now the dependency is real and explicit rather than relying on an incidental import.

Testing

• python3 -m py_compile ui.py passes.
• Behavior of the dialogs is unchanged; only the import path is made correct.
• (The repo's tests/test_ui.py mocks customtkinter and tkinter and does not exercise messagebox, so it is unaffected.)

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