Pure-function transformRgba in src/lib/image-transform.js supports flip H,
flip V, and rotation in 90° steps (fixed application order: rotate → flipH
→ flipV). Wired into the importer pipeline before resize with buttons for
each op; ±90° rotations auto-swap the resize dims so output aspect tracks
the rotation. CLI gains --flip-h, --flip-v, --rotate flags using the same
shared module.
8 new unit tests cover identity, each flip, 90/180/270 rotation, the
flipH+flipV ≡ 180° identity, and invalid rotation rejection.
