auth dialog: give the window a Doberman taskbar/title-bar icon

The approval dialog draws the Doberman mark inside the canvas (`_load_logo`, `src/doberman/auth/gui_prompter.py:153`, loads `auth/_assets/doberman-mark.png` as a stdlib `tkinter.PhotoImage`), but the window itself still carries Tk's default feather in the title bar, the Windows taskbar, and Alt-Tab. `_configure_window` (`:117`) sets the title, topmost and the dark title bar, and never calls `root.iconphoto`. There are no `iconphoto`/`iconbitmap` calls in the module. **What to do** 1. In `_configure_window` (or right after `_open_root`), load the same PNG as a `PhotoImage` and call `root.iconphoto(True, image)`. Keep a reference on `root` (the file already does `root._logo_ref = logo` at line 254 for the same reason: Tk drops unreferenced images). 2. Wrap it the way the rest of that function treats cosmetics: a failure to set the icon must never stop the dialog from opening. 3. Add a faked-root test in `tests/unit/test_gui_prompter.py` asserting `iconphoto` was called once (the file already fakes the root and canvas for the stacking and theme tests). Cosmetic; no change to what the dialog does or decides.