fix(ui): restore column drag-to-reorder functionality

[eyelock]

Summary

• Fixed column drag-to-reorder which was broken due to nested dropDestination handlers
• Column and card drops now both use String.self but are differentiated by a column: prefix
• Added onDropColumnId callback to ColumnView to handle column reordering within the same drop handler

Problem

When dragging a column over another column, the inner ColumnView's dropDestination (for cards) was consuming the drop event before the outer column reorder handler could process it. Even though the card handler returned false for invalid UUIDs, SwiftUI doesn't bubble drops up to parent views.

Solution

• Prefix column drag data with column: to differentiate from card drags
• Move column drop handling into ColumnView via a new onDropColumnId callback
• ColumnView's single dropDestination now checks for the column: prefix and routes to the appropriate handler

Test plan

• Verify column drag-to-reorder works (drag column header left/right)
• Verify card drag-and-drop still works (within same column and between columns)
• All 46 tests pass
• Zero lint violations in changed files

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