fix(auth): add auth dialog window icon

[thesageak]

Pull Request

Slice

• Repo: doberman-core
• Feature / Slice: auth dialog: give the window a Doberman taskbar/title-bar icon #432 — auth dialog taskbar/title-bar icon
• Plan reference: auth dialog: give the window a Doberman taskbar/title-bar icon #432

What this PR does

Adds the Doberman mark as the auth dialog window icon using Tk's iconphoto() API.

The icon uses the existing bundled doberman-mark.png asset and keeps a reference on the root window so Tk does not drop the image. Icon setup is best-effort and will not prevent the auth dialog from opening if loading or setting the icon fails.

Tests added (run in CI)

• Added coverage asserting _configure_window() calls iconphoto() once with the default flag enabled.
• Added coverage ensuring an icon load failure does not interrupt normal window configuration.
• tests/unit/test_gui_prompter.py: 37 passed locally.
• Ruff lint and formatting checks pass.

Public-release safety (doberman-core only)

• Contains nothing from the "not allowed" list: no enterprise/hosted code, no proprietary detection, no customer data, no secrets, no commercial-license code
• Core still builds/tests/runs with NO enterprise package installed

Security checklist

• Fails closed on error / uncertainty
• No secret, full file, or unredacted prompt logged or committed
• Any guardrail/learning change is raise-only (no silent loosening)
• Every BLOCK/AUTH carries reason codes + a human explanation
• doberman-core does not import doberman_enterprise

Edge cases covered / Deviations from plan / Risks introduced

• Icon loading is cosmetic and failure is intentionally ignored so it cannot prevent the auth dialog from opening.
• Developed and tested on Fedora with GNOME. GNOME does not display the window icon in the same way as Windows, so Windows taskbar/title-bar appearance was not visually verified.
• iconphoto() behavior is covered by the faked-root unit test.
• No changes to authorization behavior or verdict logic.

Closes #432