docs: cross-platform backend dev launcher with automatic command fallback

Add a small Node script that starts `backend/run.py` in a robust cross-platform way. It should detect and try backend launch commands in order, so Windows users can still use one command (`npm run dev`) even if `uv` is not installed but `python`/`py` is available.

Affected files: dev-backend.js

Signed-off-by: ChinhLee <76194645+chinhkrb113@users.noreply.github.com>

scripts/dev-backend.js

@@ -0,0 +1,45 @@
+#!/usr/bin/env node
+/**
+ * Cross-platform backend dev launcher.
+ * Tries launch commands in order: uv, python3, python, py
+ * so Windows users without uv can still run `npm run dev`.
+ */
+
+const { spawn } = require('child_process');
+const path = require('path');
+
+const backend_script = path.join(__dirname, '..', 'backend', 'run.py');
+
+const candidates = [
+    ['uv', ['run', 'python', backend_script]],
+    ['python3', [backend_script]],
+    ['python', [backend_script]],
+    ['py', [backend_script]],
+];
+
+function try_next(index) {
+    if (index >= candidates.length) {
+        console.error('No suitable Python interpreter found. Install uv, python3, python, or py.');
+        process.exit(1);
+    }
+
+    const [cmd, args] = candidates[index];
+    console.log(`Trying: ${cmd} ${args.join(' ')}`);
+
+    const child = spawn(cmd, args, {
+        stdio: 'inherit',
+        shell: process.platform === 'win32',
+    });
+
+    child.on('error', () => {
+        try_next(index + 1);
+    });
+
+    child.on('exit', (code) => {
+        if (code !== 0) {
+            process.exit(code);
+        }
+    });
+}
+
+try_next(0);