Dashboard Polish and Validator Error Reporting

Frontend

• Submit-result feedback on the dashboard now clears when you change or disconnect your wallet, so prior errors don't carry over to a new session.
• Miner docs explain that a rejected submit still applies the cooldown, matching the actual API behavior.
• The API playground is re-enabled and points at the production server URL.

Validator

• When a sandbox terminates abnormally, the resulting trajectory now preserves the underlying error message instead of falling back to a generic failure, so miners can diagnose the actual cause.