Allow pinboard tabs to be reordered by dragging

[brzvsk]

Summary

• make pinboard tabs draggable while keeping History fixed first
• route tab reordering through the shelf-level drop catcher so it works reliably in the non-activating glass panel
• persist the new order atomically through PinboardStore without interfering with card-to-pinboard drops
• show a before/after insertion marker during the drag

Verification

• swift test --package-path CopyCore --disable-automatic-resolution (161 tests)
• unsigned Debug app build with XcodeGen and Xcode 26.6
• manually verified reordering in the shelf and confirmed History remains fixed