fix(build): use Node.js for cross-platform file hashing in bundle script

The bundle-a2ui.sh script was using shasum which is not available on
Windows (Git Bash), causing CI failures. Replace with a Node.js-based
hash computation script that works on all platforms.

Fixes Windows CI build failures with "shasum: command not found".

Co-Authored-By: Claude Sonnet 4.5 (1M context) <noreply@anthropic.com>

scripts/bundle-a2ui.sh

@@ -30,11 +30,10 @@ collect_files() {
 }
 
 compute_hash() {
+  # Use Node.js for cross-platform hash computation (works on Windows, macOS, Linux)
   collect_files \
     | LC_ALL=C sort -z \
-    | xargs -0 shasum -a 256 \
-    | shasum -a 256 \
-    | awk '{print $1}'
+    | xargs -0 node "$ROOT_DIR/scripts/compute-file-hash.mjs"
 }
 
 current_hash="$(compute_hash)"

scripts/compute-file-hash.mjs

@@ -0,0 +1,19 @@
+#!/usr/bin/env node
+import { createHash } from "node:crypto";
+import { readFileSync } from "node:fs";
+
+// Compute SHA-256 hash of all input files (cross-platform)
+const files = process.argv.slice(2);
+const hash = createHash("sha256");
+
+for (const file of files) {
+  try {
+    const content = readFileSync(file);
+    hash.update(content);
+  } catch (err) {
+    // Skip files that don't exist or can't be read
+    process.stderr.write(`Warning: Could not read ${file}: ${err.message}\n`);
+  }
+}
+
+console.log(hash.digest("hex"));