Add fill color for closed freehand paths (+ ContextBar close/fill controls)

[akshay2211]

Description:

Summary

Element.Shape supports an optional fillColor, but Element.Path (freehand
PEN strokes) is stroke-only — there's no way to fill a loop the user has drawn.
Add a fill color for closed paths, mirroring the Shape fill model, and
expose it through the published drawbox-ui ContextBar so it's usable from the UI.

Motivation

Filling a closed freehand region is a common, expected drawing primitive, and the
data/UI plumbing for fills already exists for shapes. Today a user drawing a
closed loop can only outline it.

Design

Two additive, back-compatible fields on Element.Path:

• closed: Boolean = false — draws the closing segment (last→first) so the loop
  has a continuous outline.
• fillColor: Color? = null — interior fill, painted only when the path is
  also closed (an open path has no well-defined interior).

closed is an explicit flag (not geometric auto-detection) for determinism,
clean round-trip/SVG (Z) mapping, and collab/LWW friendliness. Closure and
fill stay orthogonal, so a closed-but-unfilled loop is also representable.

Scope

• Model: Element.Path.closed + Element.Path.fillColor.
• Renderer: fill pass under the stroke for closed+filled paths; closes both
  the uniform and variable-width stroke geometry.
• Serialization: threads both fields; legacy JSON loads as open/unfilled.
• Domain: SetSelectedFillColor now covers paths; new
  SetSelectedPathClosed intent + DrawBoxController.setSelectionPathClosed.
• drawbox-ui: ContextBarIntent.SetPathClosed, ContextBarState.pathClosed,
  ContextBarSlots.showPath, a pathContextItems builder (open/close toggle +
  fill swatch), and PathClosedIcon.
• Sample: HomeScreen treats a freehand-path selection as fillable and shows
  the close/fill chips.
• SVG export: closes (Z) and fills.
• Docs: FEATURES.md cataloging the full menu surface.

Public API note

The frozen 2.0 surface (DrawBoxController, Mode, Intent, Event, State)
is only extended here (new optional Element.Path fields + new intents), so
it's semver-safe. The drawbox-ui ContextBar additions are likewise additive.

Acceptance criteria

• A closed Element.Path with a fillColor renders filled + stroked.
• Open paths, and paths without a fill color, are unchanged.
• Fill survives JSON round-trip; legacy drawings still load.
• SetSelectedFillColor + SetSelectedPathClosed fill a selected path.
• SVG export closes and fills the path.
• Filling a path is reachable via UI: select loop → Close → Fill.

Flow

Select a freehand loop → tap Close → tap Fill → pick a color → it fills.