The dialog used hardcoded offsets to compute its target size from the
canvas's size_request:
target_w = canvas_w + 32 # 8 wrapper border * 2 + ~16 scrollbar slack
target_h = canvas_h + 80 # 8 wrapper border * 2 + 50 toolbar + slack
Two problems with that:
1. The "+32" only covered the canvas's width plus borders, not the
toolbar's width. Small layouts (e.g. an 8-LED mouse: canvas ~172px)
produced a window narrower than the toolbar wanted (~261px with
the icon buttons + palette + color picker + unset toggle), causing
toolbar overflow / clipping.
2. The "+80" assumed a fixed toolbar height and scrollbar slack —
wrong on themes with chunkier buttons or different scrollbar
metrics, and brittle to any future toolbar additions.
Replace with `wrapper.get_preferred_size()`. GTK already aggregates
the canvas's size_request through ScrolledWindow + the editor VBox +
the wrapper's border into a natural size that accounts for every
contribution, including the toolbar's width. Drop the now-unused
`canvas_size()` shim from PerKeyEditor.
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