Fix save crash when COLOR variable is missing alpha

Reuse safeColor() (which defaults a ?? 1) in the variable export
path. The Kiwi schema requires all Color struct fields — variables
created without explicit alpha caused 'Missing required field a'.

Fixes #128

packages/core/src/fig-export.ts

@@ -1,7 +1,7 @@
 import { zipSync, deflateSync, type Zippable } from 'fflate'
 
 import { CANVAS_BG_COLOR, IS_BROWSER, IS_TAURI } from './constants'
-import { sceneNodeToKiwi, fractionalPosition, buildFigKiwi, buildFontDigestMap } from './kiwi-serialize'
+import { sceneNodeToKiwi, fractionalPosition, buildFigKiwi, buildFontDigestMap, safeColor } from './kiwi-serialize'
 import { initCodec, getCompiledSchema, getSchemaBytes } from './kiwi/codec'
 import { stringToGuid } from './kiwi/kiwi-convert'
 import { renderThumbnail } from './render-image'
@@ -34,7 +34,7 @@ function variableValueToKiwi(
   }
   if (type === 'COLOR' && typeof value === 'object' && 'r' in value) {
     return {
-      value: { colorValue: { r: value.r, g: value.g, b: value.b, a: value.a } },
+      value: { colorValue: safeColor(value as { r: number; g: number; b: number; a?: number }) },
       dataType: 'COLOR',
       resolvedDataType: 'COLOR'
     }

packages/core/src/kiwi-serialize.ts

@@ -251,7 +251,7 @@ function exportTextData(node: SceneNode): NodeChange['textData'] {
   }
 }
 
-function safeColor(c: { r: number; g: number; b: number; a?: number }): Color {
+export function safeColor(c: { r: number; g: number; b: number; a?: number }): Color {
   return { r: c.r, g: c.g, b: c.b, a: c.a ?? 1 }
 }