Agent Mode: Add manual project ordering

[marksftw]

Summary

Stop reordering Agent Mode projects by recent activity. Keep the project list stable while the user works and let the user control its order by dragging project rows.

Desired behavior

• Clicking a project or one of its task sessions does not change the project's position.
• The entire project row can be dragged to a new position without breaking its existing click, expand/collapse, or new-session actions.
• During a drag, show a temporary insertion line at the exact position where the project will land.
• On drop, apply the new order and reuse the existing project-list shuffle animation as the rows settle.
• A project first added to Agent Mode appears at the top without changing the relative order of existing projects.
• The user's project order persists locally for that account and device across app launches.
• Task sessions within each project continue to use the app's existing automatic ordering.

Acceptance criteria

• Selecting a project or task session never moves its project in the list.
• A project row can be dragged above or below any other project.
• The insertion line updates during the drag and accurately previews the drop position.
• Dropping a project persists the new order and uses the existing shuffle animation.
• Canceling a drag or dropping outside a valid target leaves the saved order unchanged.
• Existing row actions remain usable and do not accidentally trigger a reorder.
• New projects appear at the top while existing projects retain their relative order.
• The saved order is restored after relaunching Maple on the same device.
• Task-session ordering remains unchanged.
• Existing reduced-motion behavior is preserved.

Reference screenshots

Four screenshots are attached to illustrate the expected resting order, new-project placement, draggable row, insertion marker, and drop animation.

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Implementation note

Project recency is currently applied both when recent roots are saved and when sidebar rows are sorted by last-used activity. Manual order should become the source of truth for projects, while the existing automatic task-session sort remains intact.

Out of Scope

• Manual ordering of Projects in Chat Mode. This can be considered as a future extension after Agent Mode.
• Changes to the automatic ordering of task sessions within Agent projects.
• Cross-device synchronization of Agent projects, manual project order, or task-session history. These remain local to the current device.