Fix GUI overflow on smaller and scaled displays

[AaronL725]

Summary

Fix the desktop GUI overflow reported in #92 without changing registration, mailbox, proxy, or output behavior.

Layout

• Put the 23-row configuration form inside a dedicated vertically scrollable viewport.
• Keep Start/Stop/Clear, status, statistics, and logs outside the scroll region so they remain visible.
• Add Windows/macOS/Linux mouse-wheel support scoped to the configuration viewport.
• Preserve CLI import behavior when Tkinter is unavailable.

Window sizing

• Replace the fixed 1120x900 / 960x700 assumptions with screen-aware initial and minimum sizes.
• Keep the GUI usable on shorter displays while retaining the existing large-screen size.

Regression coverage

• Add real Tk layout tests for 960x700 windows.
• Cover 100%, 125%, 150%, and 200% Tk scaling.
• Verify the Outlook mailbox controls remain reachable by scrolling.
• Verify mouse-wheel events do not hijack scrolling outside the configuration area.
• Run the GUI suite under Xvfb in CI on Python 3.9 and 3.13.

Fixes #92

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