Prevent running workflow during planning and when workflow is empty

[zhongkaifu]

Motivation

• Avoid situations where the UI attempts to execute a workflow while the planner is still building it.
• Provide clearer user feedback when an execution is blocked (either because planning is in progress or because the workflow is empty).
• Make the front-end more robust so users can't accidentally trigger /api/run/stream while planning is underway.

Description

• Added a planningInProgress flag in webapp/js/network.js to track when requestPlan is running.
• Set planningInProgress = true at the start of requestPlan and clear it in a finally block so it is always reset.
• Updated requestRun to guard against running while planning (planningInProgress) and to refuse execution if the current workflow is empty (isEmptyWorkflow(currentWorkflow)).
• When execution is blocked, the UI now logs a message and posts a chat message to the user explaining why the run was not started.
• Modified file: webapp/js/network.js.

Testing

• No automated tests were executed for this change.
• Manual verification: basic smoke checks in the browser were intended but not run as part of this PR.

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