feat(files): zoom and pan in the image viewer

The image viewer only had fit and 1:1, so a wide image in a narrow panel
stayed tiny and there was no step in between.

Replace the two chips with −, a percentage (click for 100%) and +, keep Fit
as the reset. Ctrl/Cmd + wheel, trackpad pinch, Safari gesture events and a
two-finger touch pinch zoom around the pointer; double-click toggles between
fit and 1:1; a zoomed image larger than the panel pans by mouse drag. Scale
math lives in imageZoom.ts.

Tested with unit tests for the scale math, ui type-check, eslint and oxlint.