Make corner resize reachable at the button corner, and widen it everywhere else

Reported live: "even where decorations are corner i should still be able
to corner resize, just its... hitbox... does not get in the way of the
close icon" - the titlebar corner holding Close/Maximize/Minimize had no
resize target at all, by design (competing with the close button was
judged worse than losing that one corner). The BUTTON_CLUSTER_MARGIN
strip between the button cluster and the frame's true edge was already
dead space no button claims, regardless of button_count - its own top
DECORATED_TOP_RESIZE_MARGIN rows now register as the diagonal corner
(TopLeft/TopRight) instead of falling through to Top/Drag, without
touching the button's own hitbox at all.

Also requested: widen the other three corners' own resize zone, since a
user reaching for a plain edge-resize instinctively aims for the middle
of that edge, not its corner - a bigger corner zone doesn't compete with
that instinct the way a bigger RESIZE_MARGIN would compete with ordinary
content clicks near an edge. CORNER_MARGIN raised from 3 to 5 (18px to
30px at the default resize_margin).

Updated one existing test whose own per-window resize_margin override
(30px) now put its plain-edge test point inside the widened corner zone
on a window too short for the two to stay apart - taller geometry, same
edge point relative to center, no change to what it actually verifies.
Added coverage for the new button-corner resize target on both sides,
and for the dead strip's own non-corner rows still just dragging as
before.

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