fix(dev-launcher): make npm run dev work on Windows

[Fronut]

Summary

npm run dev fails immediately on Windows because scripts/dev-launcher.mjs
spawns npm without a shell. This PR makes the launcher cross-platform so the
documented "From source (for developers)" quick-start works out of the box on
Windows, while leaving the macOS/Linux behavior unchanged.

Problem

On a fresh clone, following the README quick-start (npm install then
npm run dev) on Windows produces:

[dev-launcher] resolved dev ports:
  server (express/ws)  →  3001
  gateway (pilotdeck)  →  18789
  vite client          →  5173

Error: spawn npm ENOENT
    ...
  syscall: 'spawn npm',
  path: 'npm',
  spawnargs: [ '--workspace', 'ui', 'run', 'dev:concurrent' ]

Root cause:

• On Windows the npm executable is a npm.cmd shim. child_process.spawn('npm', ...)
  without a shell cannot resolve it, so it throws ENOENT.
• Simply switching to the npm.cmd name is not enough: recent Node.js releases
  refuse to spawn .cmd/.bat files without a shell and throw spawn EINVAL
  (a security hardening change related to CVE-2024-27980).

Fix

In scripts/dev-launcher.mjs, on win32 only:

• use npm.cmd as the command, and
• pass shell: true.

macOS/Linux keep the original plain, shell-free spawn('npm', ...).

The spawn arguments here are fixed string literals (--workspace, ui, run,
dev:concurrent) with no user input, so enabling the shell on Windows carries
no command-injection risk.

const isWindows = process.platform === 'win32';
const npmCommand = isWindows ? 'npm.cmd' : 'npm';
const child = spawn(
  npmCommand,
  ['--workspace', 'ui', 'run', 'dev:concurrent'],
  { cwd: repoRoot, env, stdio: 'inherit', shell: isWindows },
);

Testing

Verified on Windows (PowerShell, Node.js v22.14.0, npm 10.9.2):

• npm run dev now boots all three processes successfully:
  • vite client → http://localhost:5173 (HTTP 200)
  • server (express/ws) → http://localhost:3001 (HTTP 200, "PilotDeck Server - Ready")
  • gateway (pilotdeck) → ws://127.0.0.1:18789/ws (listening)
• No ENOENT / EINVAL in stderr.

macOS/Linux code path is unchanged (still plain spawn('npm', ...) with no shell).

Scope

This PR intentionally fixes only the developer quick-start entry point
(dev-launcher.mjs), which is the one blocker that prevents npm run dev from
starting at all on Windows. Other spawn('npm'|'npx', ...) call sites in
runtime/optional feature paths are out of scope here and can be addressed
separately if desired.