Pinch-to-zoom in lightbox (mobile)

[Darkkal]

Parent Issue

Part of #74 — More controls for media in lightbox

Problem

On mobile, users expect to pinch-to-zoom on images just like in native photo viewers. The zoom infrastructure from the fit mode & zoom sub-issue provides the scale/pan state, but touch gesture recognition needs dedicated handling.

Proposed Design

• Two-finger pinch gesture on images enters zoom mode (if in fit mode) and adjusts zoom level
• Uses the same lightboxZoomMin / lightboxZoomMax bounds from settings
• Zoom anchors to the midpoint between the two fingers
• Pinch gestures are distinguished from single-finger swipe (navigation) by tracking touch count
• When zoomed in via pinch, single-finger drag pans the image (swipe-to-navigate disabled)
• When zoom returns to or below fit level, fit mode re-engages and swipe-to-navigate resumes

Acceptance Criteria

• Two-finger pinch-in zooms out, pinch-out zooms in
• Zoom is bounded by lightboxZoomMin and lightboxZoomMax from settings
• Zoom anchors to the midpoint between the two touch points
• Pinch gestures do not trigger swipe navigation
• Single-finger pan works when zoomed in (swipe-to-navigate disabled)
• Releasing pinch below fit-level snaps back to fit mode and re-enables swipe navigation
• Zoom percentage overlay appears during pinch zoom
• Videos are excluded from pinch-to-zoom
• Zoom state resets on navigation to next/prev item

Edge Cases

• Rapid alternation between one-finger and two-finger touches — debounce gesture detection to avoid flickering between pan and navigate modes
• Pinching while the bottom sheet sidebar is open — pinch should only activate on the media area, not the sidebar
• Very small pinch movements (< threshold) should be ignored to prevent accidental zoom

[Darkkal]

implemented and validated in #132