Give corner resize real priority over a plain edge

Reported live, repeatedly: corners felt like they had no priority over
sides. They didn't, structurally - a corner only ever registered in
the exact pixel square where both edges' own resize_margin zones
happened to overlap (6x6px at the default 6px margin), nothing wider.
A click a little further along either axis, still clearly aiming for
the corner, fell back to a single straight-edge resize instead.

resize_edge_at now checks a separate, wider corner zone
(CORNER_MARGIN * resize_margin) first, so a corner claims a real,
deliberately larger target the way GNOME/KDE already do - diagonal
resize is a harder grab than a straight edge and deserves more room,
not the same or less.

Also fixed a related, previously-unreachable case: a *decorated*
window's whole titlebar band returned Drag/Close/Maximize/Minimize
unconditionally, so top-left/top-right diagonal resize never had a
code path at all, even at the titlebar's own corner pixels. Added
top-left resize as a small, genuine corner square (both axes, not just
one) checked before drag/buttons. Deliberately did NOT add the
matching top-right case: that corner is the close button on every
mainstream desktop, and trading a well-known, expected target for a
rarely-wanted one at exactly the spot a miss costs the most isn't a
trade worth making.

Four new regression tests cover: reaching a corner past the old tight
margin, falling back to a plain edge just past the new wider one, the
newly-reachable decorated top-left corner, and confirming top-right
still closes rather than competing with resize.

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