Redesign pgschema icon as paired delta diff

Replace the 2x2 grid with two opposing deltas (an up triangle + a down
triangle) = two schema states being compared, on one shared red
gradient — completing the triangle-based icon family while keeping a
silhouette distinct from the single-triangle siblings. Drop the unused
stale pgschema.png raster.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tianzhou