fix(ci): replace bash prebuild with cross-platform Node.js script

The prebuild script used bash syntax (mkdir, for, [ -f ]) which fails on
Windows. Replace with a Node.js script that checks platform() and skips
non-Linux, keeping the plugin copy logic only where it's needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

openless-all/app/package.json

@@ -6,7 +6,7 @@
   "scripts": {
     "dev": "vite",
     "build": "tsc && vite build",
-    "prebuild": "mkdir -p src-tauri/linux-fcitx5-plugin && for f in libopenless.so openless.conf; do [ -f ../scripts/linux-fcitx5-plugin/build_release/$f ] && cp ../scripts/linux-fcitx5-plugin/build_release/$f src-tauri/linux-fcitx5-plugin/; done; true",
+    "prebuild": "node scripts/copy-plugin.mjs",
     "preview": "vite preview",
     "tauri": "tauri",
     "check:hotkey-injection": "node scripts/check-hotkey-injection.mjs"

openless-all/app/scripts/copy-plugin.mjs

@@ -0,0 +1,27 @@
+import { copyFileSync, mkdirSync } from 'fs';
+import { join, dirname } from 'path';
+import { fileURLToPath } from 'url';
+import { platform } from 'os';
+
+// Only relevant on Linux — skip silently on other platforms
+if (platform() !== 'linux') {
+  process.exit(0);
+}
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const srcDir = join(__dirname, '..', 'scripts', 'linux-fcitx5-plugin', 'build_release');
+const destDir = join(__dirname, 'src-tauri', 'linux-fcitx5-plugin');
+
+const files = ['libopenless.so', 'openless.conf'];
+
+for (const f of files) {
+  const src = join(srcDir, f);
+  const dest = join(destDir, f);
+  try {
+    mkdirSync(destDir, { recursive: true });
+    copyFileSync(src, dest);
+    console.log(`[copy:linux-plugin] copied ${f}`);
+  } catch {
+    console.log(`[copy:linux-plugin] ${f} not found — skipping`);
+  }
+}