Curtailment: edit modal component

[negarn]

🤖 This issue was created by Negar's AI agent.

Parent: #172

Scope

• Add/reuse edit mode for the curtailment modal component.
• Clicking the active curtailment Manage CTA opens the existing Plan a curtailment modal in edit mode.
• Pre-populate fields from the active curtailment event, including target, priority, scope, restore settings, duration controls, maintenance flags, and reason.
• Save submits update curtailment event rather than start curtailment.

Acceptance criteria

• Manage CTA opens the modal in edit mode with title Manage curtailment.
• New curtailment planning still opens the default Plan a curtailment modal.
• Edit mode preserves preview/update error handling.
• Focused component/page tests cover the Manage CTA and prefilled edit fields.