fix: painter widget preserves mask reference across WidgetPainter remount

The painter's serializeValue closure captured hasStrokes and isDirty as
local variables that reset on every WidgetPainter.vue mount. After a
remount (e.g. NodeWidgets re-render driven by useProcessedWidgets, added
in #10966), serializeValue saw isCanvasEmpty()=true and returned '' even
when modelValue still held a valid mask reference from a workflow load
or a prior upload — backend then received an empty mask, and on cloud
this surfaced as ImageDownloadError.

Reorder the short-circuits so a non-dirty painter returns its existing
modelValue.value before the canvas-empty guard, and fall back to the
prior modelValue.value if the canvas element is no longer mounted.

src/composables/painter/usePainter.test.ts

@@ -359,15 +359,14 @@ describe('usePainter', () => {
       expect(result).toBe('')
     })
 
-    it('returns empty string when canvas has no strokes even if modelValue is set', async () => {
+    it('returns existing modelValue when not dirty (regression: WidgetPainter remount must not blank a workflow-restored mask reference)', async () => {
       const maskWidget = makeWidget('mask', '')
       mockWidgets.push(maskWidget)
 
-      const { modelValue } = mountPainter()
-      modelValue.value = 'painter/existing.png [temp]'
+      mountPainter('test-node', 'painter/existing.png [temp]')
 
       const result = await maskWidget.serializeValue!({} as LGraphNode, 0)
-      expect(result).toBe('')
+      expect(result).toBe('painter/existing.png [temp]')
     })
   })
 

src/composables/painter/usePainter.ts

@@ -624,13 +624,13 @@ export function usePainter(nodeId: string, options: UsePainterOptions) {
   }
 
   async function serializeValue(): Promise<string> {
+    if (!isDirty.value) return modelValue.value
+
     const el = canvasEl.value
-    if (!el) return ''
+    if (!el) return modelValue.value
 
     if (isCanvasEmpty()) return ''
 
-    if (!isDirty.value) return modelValue.value
-
     const blob = await new Promise<Blob | null>((resolve) =>
       el.toBlob(resolve, 'image/png')
     )