[feature]: Add support for Synthetic provider

[eduardhildebrandt]

Pre-flight checks

• I searched existing issues and did not find a duplicate request.
• I reviewed current behavior on the production released OpenCode version.

OpenCode version reviewed

1.14.18

Problem statement

Synthetic is a native provider for OpenCode. The plugin currently does not support quotas for Synthetic.

Proposed change

Please add support for Synthetic provider.

Documentation of how to check quotas via REST API can be found here:
https://dev.synthetic.new/docs/synthetic/quotas

Documentation on how to use OpenCode with Synthetic can be found here:
https://dev.synthetic.new/docs/guides/opencode

Alternatives considered

No response

Acceptance criteria

• Remaining quotas for Synthetic are displayed in the sidebar the same way they are currently displayed for OpenAI.

[slkiser]

Added in v3.1.0