feat: add Gemini provider to usage tracker

[nora-ellis-ai]

Adds Gemini (Google AI Studio) as a provider in the Usage Overview modal.

Google AI Studio does not expose a programmatic billing or usage endpoint, so this implementation validates the API key with a lightweight GET to the v1beta/models endpoint and displays connection status only.

What this adds:

• fetchGeminiUsage() following the existing provider pattern
• Missing key shows missing_credentials status
• Invalid or expired key shows auth_expired
• Valid key shows "API key active" and "Usage data: Not available via API"
• Registered in Promise.allSettled() aggregate — failure does not affect other providers

To use: add GOOGLE_API_KEY to your hermes-workspace .env file.