feat(viewer): smooth ViewPager2 paging + magicpro default v1.091

User feedback: paging through multi-page scans felt awkward, and asked
for Magic Pro to be the default filter.

- Multi-page viewer now uses ViewPager2 instead of a custom fling on
  ZoomableImageView. Pages drag-swipe smoothly with a page-count pill in
  the top bar (back button + title). ZoomableImageView yields horizontal
  swipes to the pager at min zoom and only claims the gesture (panning)
  when zoomed in or while a pinch is in progress, so zoom and paging no
  longer fight. CamPagerAdapter holds one zoomable image per page and
  refreshes only the visible page after in-place edit/rotate.
- Magic Pro is the default filter across all three paths: camera preset,
  editor fallback (import / re-edit), and the batch ScanPipeline (which
  now runs ProFilters.magicPro instead of magicColor+sharpen+brighten),
  so saved scans get the pure-white, crisp look by default.

Constraint: ZoomableImageView consumes the gesture only when zoomedIn or pinching; otherwise the pager pages. ProFilters.magicPro never recycles its source.
Confidence: high
Scope-risk: module
Reversibility: moderate
Directive: broader app-wide visual polish (main list, action card, theming) is the next pass; on-device, confirm pinch-zoom does not page-flip.
Tested: assembleDebug + testDebugUnitTest exit=0 (22 tests); reviewer pass after fixing the pinch/VP2 race and nullable RecyclerView cast.
Not-tested: on-device gesture feel and magicpro look across papers.
Related: CamScanner libmagicenhancer analysis (MNN-based, encrypted — confirmed no classical recipe to port)

🐙 Autopus <noreply@autopus.co>