feat(editor): live filter-preview thumbnails v1.093

User liked CamScanner's filter strip where each filter is a live preview
of the actual document. Replaced the editor's text filter buttons with a
horizontal strip of preview thumbnails.

- FilterPreviews.generate downscales the flattened page to ~220px and
  applies every filter once, off the main thread, yielding one thumbnail
  per filter (original is copied so its buffer is owned independently).
- FilterStripAdapter renders each thumbnail in a small rounded card with
  its label; the selected filter gets a brand border + brand label.
- EditorActivity builds the strip in showFilterStage and selects the
  default (magicpro). Tapping a thumbnail applies the full-resolution
  filter to flattened (unchanged quality path) and moves the selection.
- Removed the old text-button wiring and updateFilterHighlight.

Constraint: thumbnails are preview-only (220px); the chosen filter is always re-applied to the full-res flattened bitmap. small preview base is recycled; per-filter thumbnails are held by the adapter for the activity's life.
Confidence: high
Scope-risk: module
Reversibility: moderate
Directive: next — show batch/multi-import as a grid with per-page progress (per user reference shots).
Tested: assembleDebug + testDebugUnitTest exit=0 (22 tests); manual audit of preview bitmap ownership/recycle.
Not-tested: on-device strip scroll/look and preview generation latency on a phone.
Related: CamScanner single-image editor reference (filter thumbnail strip)

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