[fylorn]

Summary

• Opening size no longer follows the interface size. As in a browser, zoom enlarges the content, not the window. The main window opens at 1100×720 pt at every step. It grows only when the scaled minimum size is larger than that, which happens only at 150% (1230×840 pt, still clamped to the screen's work area). zoom::main_sizes returns both sizes.
• Always centered. The window position is not remembered. Without center(), Windows placed it at the cascade position near the top left. It was centered before only when the clamped size filled one side of the screen.
• Changing the size while the window is open works as before: only the minimum size changes, and the window grows if it is smaller.

Test plan

• cargo fmt --check, cargo clippy --all-targets -- -D warnings
• cargo test: new the_opening_size_stays_unless_the_minimum_needs_more; all unit tests pass

Closes #376

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