feat(webui): toggle detail panel by re-clicking the active card

[jimpablo]

Summary

• Clicking an already-selected envelope card now closes the detail panel instead of keeping it open
• Widens onSelect prop type from (id: string) to (id: string | null) in Column and KanbanBoard

Test plan

• Click an envelope card → detail panel opens
• Click the same card again → detail panel closes
• Click a different card while one is open → switches to new card's detail
• X button still closes the panel as before

[Copilot]

Pull request overview

This PR updates the WebUI inbox interaction so that clicking the currently-selected envelope card toggles the detail panel closed, making selection behavior more consistent with a “toggle” interaction pattern.

Changes:

• Allow onSelect to receive null in KanbanBoard and Column prop types so selection can be cleared via the same callback.
• Implement “re-click active card to close” by toggling selection to null when the clicked card is already active.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
loom/webui/frontend/src/components/KanbanBoard.tsx	Widens onSelect prop typing to accept null to support clearing the selection.
loom/webui/frontend/src/components/Column.tsx	Updates onSelect typing and toggles selection off when clicking the active EnvelopeCard.

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