Add warning for `dev + --mode production` usage in vite-plugin-monkey

[kaixinol]

When using vite-plugin-monkey, running:

pnpm dev --mode production

can cause runtime issues such as:

• [vite-plugin-monkey] not found monkeyWindow
• GM_getValue is not a function

Although this is valid in Vite, it breaks the dev server + userscript bridge in vite-plugin-monkey.

It would be helpful to show a warning when detecting serve + production mode, e.g.:

⚠️ Avoid using `dev` with `--mode production`. Please use:

• pnpm dev (development)
• pnpm build (production)