fix(Util): preserve alpha of the named color "transparent"

[spokodev]

Problem

Konva.Util.colorToRGBA('transparent') returns an opaque color:

Konva.Util.colorToRGBA('transparent');
// { r: 255, g: 255, b: 255, a: 1 }   ← a should be 0

The named-color table already defines it correctly with a zero alpha:

// src/Util.ts
transparent: [255, 255, 255, 0],

but _namedColorToRBA hard-codes a: 1 and ignores the fourth element:

return { r: c[0], g: c[1], b: c[2], a: 1 };

Every other parser honors alpha — #0000 and rgba(0,0,0,0) both correctly
return a: 0. Named colors are the only branch that drops it, and
transparent is the only named color that carries a non-1 alpha, so it is the
one value this branch gets wrong.

Impact

colorToRGBA feeds Tween color interpolation (fill / stroke / shadowColor).
Tweening to or from 'transparent' — the canonical fade in/out idiom —
computes an alpha delta of 1 - 1 = 0, so the shape never fades; it snaps to
fully opaque white (255, 255, 255) instead of fading out.

Fix

Honor the alpha in the color table, mirroring the hex/rgba parsers:

a: c.length > 3 ? c[3] : 1,

Test

Added a colorToRGBA() case asserting transparent → a: 0 (and red → a: 1
so the default-alpha path stays covered). Fails on master, passes with the fix.

[spokodev]

Heads-up: the failing fmt-check here is unrelated to this PR — it flags test/unit/DragAndDrop-test.ts, which this change doesn't touch and which is already unformatted on master, so prettier --check . currently fails on any branch. This PR's own files (src/Util.ts, test/unit/Util-test.ts) pass prettier --check.