Add pinch/pan/double-tap zoom to the full-screen photo viewer, mirroring the
web viewer semantics:
- scale is a multiplier of the natural size, capped at 200%; the fitted view
is the lower bound
- double tap toggles fitted view <-> 100% natural anchored at the tap
- the original loads on zoom, with the preview kept as a placeholder so
entering zoom never flashes a black surface
- the pager is disabled while zoomed (drags pan the image) and the zoom resets
when leaving a photo
- video and 360 panoramas are unaffected
Implement PhotoZoom (zoom math) and ZoomableImage (gestures), wire it into
ViewerContent/MediaPage, and cover it with unit, Compose and device tests.
OpenSpec change: add-mobile-photo-zoom
