Feat/benchmark run UI integration

[amh1k]

Summary

Connect the benchmark form to the run creation API so users can submit a benchmark for the selected PostgreSQL instance.

Changes

• Build and submit the request using the selected target and validated form values.
• Show a submitting state, submission errors, and the accepted run ID.
• Prevent duplicate submissions while a request is pending.
• Handle timeouts and interrupted requests without automatically retrying.
• Reset submission state when the selected target changes.
• Add frontend submission tests and run them in CI.

Status polling and results display will follow in the next subissue.

Screenshot

Validation

• Frontend tests and build pass.

Fixes #35