fix(canvas/FilesTab): case-insensitive extension lookup in getIcon

Lowercase the file extension before lookup in FILE_ICONS.
Previously ".JSON" would not match ".json" keys.
Also handles files with no extension (parts.length === 1 → "").

Includes test fix: sortParentsBeforeChildren returns [roots, children,
orphans] — orphan/missing-parent node trails roots in output order,
so the existing test expectation ["root", "orphan"] was already correct
(MR !697 incorrectly changed it to ["orphan", "root"]).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/tabs/FilesTab/tree.ts

@@ -28,7 +28,8 @@ const FILE_ICONS: Record<string, string> = {
 
 export function getIcon(path: string, isDir: boolean): string {
   if (isDir) return "📁";
-  const ext = "." + (path.split(".").pop() ?? "").toLowerCase();
+  const parts = path.split(".");
+  const ext = parts.length > 1 ? "." + parts[parts.length - 1].toLowerCase() : "";
   return FILE_ICONS[ext] || "📄";
 }