Draw by drag

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[cornhundred]

Draw neighborhoods by dragging (freehand SKTCH mode)

Replaces the click-to-place-vertices polygon tool with freehand drag-to-draw,
making neighborhood sketching faster and more natural, and adds keyboard
shortcuts to enter/exit sketch mode.

Changes

• Freehand drawing — SKTCH now uses DrawPolygonByDraggingMode instead of
  DrawPolygonMode; drag to trace a region, release to finish. Sampling is
  throttled to ~60fps (throttleMs: 16).
• Keyboard shortcuts — press s to enter sketch mode and Escape to exit.
  The shortcut is scoped to whichever widget the pointer is hovering (checked
  via :hover at keydown time), so multiple Landscape widgets on one page
  (e.g. a notebook) don't all toggle at once. Guarded against modifier keys
  and input/editable targets so it won't fire while typing, and the listener
  is registered in the capture phase and calls stopPropagation() when it
  acts, so it claims the keystroke before it can reach host-page shortcuts
  (e.g. Jupyter's own s-to-save binding in command mode). Sketch mode is
  single-shot — a completed stroke auto-returns to view mode.
• Refactor — extracted the SKTCH enter/exit logic into a single
  set_sketch_mode() helper shared by the button and the keyboard shortcut,
  and made the NBHD-edit-visibility toggle (which forces the layer/button
  back to view state independently of that helper) reset viz_state.edit.mode
  in lockstep, so the two can't drift out of sync.
• Lifecycle — the keydown listener is torn down in landscape_ist.finalize(),
  and make_ist_ui_container always cleans up any prior listener on the same
  viz_state before conditionally re-registering (not just when NBHD edit is
  currently on), so re-running it can't orphan a handler.
• Perf — addTentativePosition / cancelFeature edits short-circuit to a
  layer refresh, skipping add-feature finalization during a drag; that refresh
  now passes viz_state through to get_layers_list like every other call
  site, so point-cloud layer filtering stays correct on that path.

Files

• js/ui/ui_containers.js — set_sketch_mode, shortcut handler, hover-scoping,
  listener cleanup
• js/deck-gl/layers/edit_layer.js — modeConfig support, tentative-position fast path
• js/viz/landscape_ist.js — edit.mode kept in sync on NBHD-edit toggle, shortcut cleanup on finalize

Notes

• Merged main (0.18.0, set/collection work) into this branch; version bumped to 0.19.0.
• Bundle docs/assets/js/celldega.js regenerated via npm run build.

Known limitation / follow-up

• Mode values ('sktch'/'view'/'modify') are bare string literals shared
  across files; consolidating them into shared constants is a pre-existing
  cleanup left for a separate PR.