feat(batch): visible grid with per-page progress v1.094

User liked CamScanner showing batch/multi-import as a grid with a
per-page spinner instead of a blocking modal dialog.

- New BatchGridActivity: shows every source page in a 3-col grid up
  front, each cell with a spinner + page number, and processes them one
  by one (detect -> flatten -> magicpro) updating each cell to its result
  as it finishes. Header counts down remaining; back is blocked until
  done; then a share/save/done bar appears and tapping a page opens the
  pager viewer.
- BatchGridAdapter holds per-cell state (source thumb + spinner -> result
  thumb). MediaSaver.saveBitmapToGroupUri returns the saved MediaStore
  URI so the grid can collect and act on results (saveBitmapToGroup now
  delegates to it).
- Both entry points use it: shared multi-image (MainActivity) and
  multi-capture finish (MultiCapture) now launch BatchGridActivity.
- Removed the now-unused BatchProcessor and MainActivity's modal-progress
  code (orphaned by this change).

Constraint: pages process sequentially so saved.size is read consistently between iterations; back is disabled mid-processing to avoid a half-saved group.
Confidence: high
Scope-risk: module
Reversibility: moderate
Directive: model research (open deshadow) conclusion pending — DocShadow tested worse than classical, GCDRNet has no license, DocRes is MIT but a heavy generalist needing manual weight setup.
Tested: assembleDebug + testDebugUnitTest exit=0 (22 tests); manual audit of coroutine ordering, bitmap recycle, orphan cleanup.
Not-tested: on-device batch run look and FileProvider pending-thumb fallback.
Related: CamScanner batch grid reference

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