fix(editor): stop UVDoc skewing flat docs; bottom sheet no longer covers preview v1.095

User report on a straight photo with correct corners: the result came out
as a slight parallelogram, and the bottom buttons covered the document.

Skew fix:
- UVDoc dewarp was applied unconditionally after the perspective crop. On
  an already-flat, well-cropped page UVDoc's grid is never identity, so it
  introduced a shear. Spike confirmed the grid's nonlinearity does not
  cleanly separate flat from curved, so an auto-gate is unreliable.
- Dewarp is now opt-in: EditorFlatten.run takes a dewarp flag; default is
  off (clean perspective crop, no skew). A "펴기" toggle in the editor
  toolbar turns UVDoc on for genuinely curled/curved pages (brand-tinted
  when active, snapshotted before the background job, disabled while busy).
- ScanPipeline (batch) drops UVDoc too — perspective crop only — so
  batch-processed flat docs aren't skewed either.

Layout fix:
- The editor bottom sheet was a fixed 148dp while the filter stage
  (thumbnail strip + brightness + done button) is taller, so it overlapped
  the preview. Reworked activity_editor.xml: root is now a vertical
  LinearLayout with the preview area as weight=1 (preview/overlay/loupe/
  busyOverlay fill it, margins removed) and the sheet as wrap_content, so
  the sheet never covers the document regardless of height. Overlay and
  preview stay identically sized, preserving corner-coordinate mapping.

- Extracted the completion/ID-finalization logic to EditorFinish to keep
  EditorActivity under the 300-line limit (now 263).

Constraint: overlay and preview share one weight=1 FrameLayout so they remain pixel-identical for corner mapping; dewarp flag is snapshotted before dispatch.
Confidence: high
Scope-risk: module
Reversibility: moderate
Directive: on-device, confirm a flat page comes out perfectly rectangular by default and "펴기" still flattens a curled page.
Tested: assembleDebug + testDebugUnitTest exit=0 (22 tests); reviewer pass after the file-split; UVDoc grid nonlinearity spike.
Not-tested: on-device skew and the new toggle visual state.
Related: user receipt skew report

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