Release v0.2.1: add-device modal polish

[scott-tp]

Summary

• Fix modal backdrop changing color on hover (backdrop is a <button>; global button:hover no longer overrides the dimmer).
• Use consistent add-device copy: top bar Add a device, modal title Add a TinyPilot device, submit Add device.
• Center the submit button and add spacing below the API key field.

Version

Bumps dashboard version to 0.2.1.

Test plan

• Open dashboard with no devices — modal auto-opens with title Add a TinyPilot device
• Hover modal backdrop — overlay stays a steady dim color (light and dark mode)
• Submit button is centered with clear spacing below API key
• Top bar shows Add a device; submit shows Add device
• GET /api/version returns 0.2.1