Fix #400: Scrub numeric fields by dragging their label

Dragging a numeric field's label left or right steps the field's value,
as Photoshop's scrubby labels do (beyond Illustrator): one unit every
2 points, ten with Shift, a tenth with Ctrl/Cmd (counts stay whole), in
the field's unit and precision. The panels apply each value as they
apply a typed one, so the art follows live and the caller's limits
hold (dragging back from a limit changes the value at once).

- scrub.rs: the label zone over the label noted just before a field
  (dim_label, label_row, widgets::field_label, the Character and
  Paragraph panels' icons, the Transform and Properties rotate/shear
  icons), the ↔ cursor on hover, a click on the label focuses the
  field. The text box itself never scrubs; a field without a label
  doesn't. num_field, plain_field, mixed_field (and the spin fields)
  and opt_field take part.
- Engine: undo groups (Session::begin_undo_group / end_undo_group). The
  edits made in a group, commands and committed live drags alike, are
  one undo step, journaled one by one; a cancelled group is undone and
  dropped from the journal. The app opens one while a label is dragged,
  so a scrub is one undo step whichever path the panel applies values
  by (app.run, session.execute, live_run), and Escape cancels it
  (shortcuts leave Escape to the drag, so it doesn't close a dialog or
  deselect).
- Preferences > General > Scrub Numeric Fields by Dragging
  (`scrubNumericFields`, on by default), translated in ja, es, zh-hans,
  zh-hant and pt-br.
- Tests: label drags with Shift/Ctrl, limits, counts and units, Escape,
  the preference, the cursor, click-to-focus, no scrub on the box or
  without a label, and in the app one undo step for a W drag.
- ROADMAP and docs/mcp.md.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/mcp.md

@@ -1147,7 +1147,9 @@ The Selection & Anchor Display and General preferences apply to `pointer_gesture
 The preferences for the view need the desktop app (`vectorcraft-cli mcp --connect`): `zoomWithMouseWheel` (the
 wheel zooms about the pointer, Shift-wheel scrolls up and down, Cmd/Ctrl-wheel sideways; the control channel's
 `ui.wheel` turns the wheel), `zoomToSelection` (on: Zoom In and Zoom Out centre the selection), `showToolTips`,
-`anchorSize` (1–7), `gridColor`, `gridStyle`, `gridsInBack`, `guideColor`, `guideStyle` and `recentFontsCount`.
+`anchorSize` (1–7), `gridColor`, `gridStyle`, `gridsInBack`, `guideColor`, `guideStyle`, `recentFontsCount` and
+`scrubNumericFields` (on: a horizontal drag on a numeric field's label steps the field, one undo step per drag; the
+control channel's `ui.drag` scrubs).
 
 ## Type preferences