Add controller for running a workflow and viewing results

### Feature Summary

Step 5 of the operator-demo video generator: add a controller that runs the workflow and opens the result panel, so the demo ends on the output a user can expect.

### Proposed Solution or Design

Add a new ExecutionControllerBuilder to the Playwright layer. It enables result viewing on the operator, runs the workflow, waits for the run to finish, and opens the result panel.

```
new ExecutionControllerBuilder(ctx)
.enableViewResult()
.runWorkflowAndWait()
.openResultPanel()
.execute()
```
The controller handles:
- Starting the run: waits until the Run button is usable, optionally creates or selects a computing unit, and uses a brief press-and-hold when triggering the run.
- Waiting for completion: first waits for the run to leave the idle state, then waits for execution to finish with a hard timeout on failure.
- Validating results: opens the result panel and verifies that it contains valid output rather than an empty result or error state.

Frontend hooks: None expected: the Run button already has a stable id.

### Affected Area

_No response_