fix(shell): persist drag-and-drop pane reorder across navigation

[DannyNs]

Summary

• The frontend emitted state:reorder-panes on drag-and-drop but the server had no socket handler, so dashboardState.panes was never reordered
• When navigating away and returning to the Shell view, the snapshot restored the original (pre-drag) order
• Added a server-side state:reorder-panes handler in shell-socket.ts that validates the incoming order, reorders the panes array, and broadcasts to other clients

Test plan

• Open Shell view, create 3+ terminal panes
• Drag-and-drop to reorder panes
• Navigate to another module (e.g. Kanban) and back to Shell
• Verify pane order is preserved
• Open a second browser tab and verify reorder syncs across clients

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