#92: shrink the confirm popup to the window it is in

The card is authored at a fixed 704x1200 around a portrait phone. On a
window wider than it is tall the stretched canvas is about 1080 units high,
so the card sat at y=-60 and hung over both edges - and the confirm and
cancel buttons are anchored to its own bottom edge, so they went off screen
with it. The popup was not merely ugly on a desktop, it was a dead end:
nothing on it could be pressed.

NeonPanel now fits itself to the viewport and re-fits when the window
changes. It only ever shrinks; growing the card would stretch proportions
that are tuned rather than fluid. The fitting adjusts offsets around a
fixed centre, so it applies only to a card pinned to a point - a panel
stretched between different anchors already follows its parent and is left
alone.

Verified across portrait, 16:9, ultrawide and small-laptop shapes, on the
card and on both buttons.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_build_popup_fits_window.gd.uid

@@ -0,0 +1 @@
+uid://bt0yr2oys1usk