feat(image): preserve transparency for alpha images end to end

Skip WebP re-encoding for PNG/AVIF/BMP uploads and any bitmap that
actually contains alpha, tag transparency-capable canvas uploads with
the transparent-image tone, and detect alpha in merged layers so the
composed node keeps a transparent tone.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/pages/CanvasWorkspace/index.tsx

@@ -3095,6 +3095,7 @@ export function CanvasWorkspace({ projectId }: { projectId: string }) {
         const previewUrl = URL.createObjectURL(file);
         const dimensions = await readImageDimensions(file).catch(() => ({ width: 512, height: 512 }));
         const size = fitCanvasImageSize(dimensions.width, dimensions.height);
+        const tone = imageMaySupportTransparency(file) ? "transparent-image" : "natural";
         preparedUploads.push({
           file,
           previewUrl,
@@ -5095,6 +5096,23 @@ function isImageFile(file: File) {
   return file.type.startsWith("image/") || /\.(png|jpe?g|webp|gif|avif|svg)$/i.test(file.name);
 }
 
+function imageMaySupportTransparency(file: File) {
+  const type = file.type.toLowerCase();
+  const name = file.name.toLowerCase();
+  return (
+    type.includes("png") ||
+    type.includes("webp") ||
+    type.includes("avif") ||
+    type.includes("gif") ||
+    type.includes("svg") ||
+    name.endsWith(".png") ||
+    name.endsWith(".webp") ||
+    name.endsWith(".avif") ||
+    name.endsWith(".gif") ||
+    name.endsWith(".svg")
+  );
+}
+
 function dataTransferImageFiles(dataTransfer: DataTransfer, fallbackBaseName = "canvas-image") {
   const candidates = [
     ...Array.from(dataTransfer.files),