0.12.0 — Text tool, point editing, and the edit toggle

Breaking

• drawing_mode="edit" (formerly "transform") is removed. Editing is now the
  toolbar's edit toggle, on in every mode -- there is no argument for it. Passing
  "edit" or "transform" now raises ValueError. There is no replacement argument:
  an edit-only canvas is no longer expressible this way -- use disabled=True for a
  read-only canvas instead.
• Polygons now serialize as {type: "Polygon", points: [...]} instead of
  {type: "Path", path: [...]}. A closed (M/L/Z) Path in initial_drawing is
  converted to an equivalent Polygon on load, so a display-only canvas can return a
  different payload than the one it was given.
• Dragging a rect's corner in edit mode's point editing converts it to a Polygon.
  A rect stays a Rect for any other interaction, including point editing's other three
  corners without a drag.

Added

• drawing_mode="text": click to place a Fabric IText and start typing
  immediately. Nothing is sent to Streamlit until you click away (or Escape/blur) --
  one undo removes the whole text object, not one keystroke. New font_size: int = 20
  parameter, ignored outside text mode. With the toolbar's edit toggle on, click an
  existing text object to select it, then click it again (a second, separate click) to
  re-enter editing -- a fast double-click doesn't reliably trigger it, a pre-existing
  Fabric quirk.

• Point editing, edit mode's second level. Select a polygon, line, rect or circle,
  then click it again (same gesture as re-entering text) to edit its individual points
  instead of moving/scaling/rotating the whole shape. No new parameter -- it's part of
  the toolbar's edit toggle.

  Shape	Drag a handle...	Click a handle...
  Polygon	moves that vertex	removes it (floor: 3 vertices)
  Line	moves that endpoint	--
  Circle	sets the radius from that rim point	--
  Rect	converts it to a Polygon, then moves that corner as a vertex	--

  Excluded from point editing: freedraw Paths, any object with a lock* property set,
  multi-selections, and non-uniformly-scaled circles.

Fixed

• Clicking to place text near the canvas edge could scroll the canvas out of view,
  leaving it blank. Fabric positions IText's hidden input textarea assuming it's
  parented on doc.body (page-absolute coordinates); reparenting it to stay inside the
  shadow root (needed since 0.10.0's isolate_styles=True) made that math double-count
  the canvas's own page offset, placing the textarea far outside the canvas. Focusing it
  then made the browser auto-scroll the canvas's internal scroll container to reveal it.
  Fixed with a dedicated zero-size, clipped anchor element outside that scroll container.

Changed

• fill_color now defaults to None (was "#eee"), resolved per mode: shapes
  (rect/circle/polygon) still get "#eee"; text gets stroke_color instead, since
  "#eee" text on a default canvas would be all but invisible. Passing fill_color
  explicitly behaves identically to before in every mode.
• Polygon is reworked: every vertex now shows a visible handle. Click the first
  vertex's handle to close the shape (needs at least three vertices); click any other
  handle to remove that vertex. This replaces the double-click-removes-last-points
  gesture and, completing the 0.11.0 right-click cleanup, right-click no longer
  closes the polygon -- it has no special meaning in any mode now. The completed
  polygon still always sends once closed, regardless of update_streamlit.
• Draw-time polygon vertex handles are a fixed 10px radius instead of scaling with
  stroke_width, matching point editing's handle size.