Feature/drag to reorder applets

[boloto1979]

Summary

Implements drag-to-reorder for applets in the COSMIC panel, similar to
the long-press reorder in macOS menu bar.

Behavior

• Hold an applet for 400 ms to enter drag mode
• While dragging, other applets slide smoothly to show a live preview
  of the new position (ease-out-cubic animation, 180 ms)
• Release to commit the new order; it is persisted immediately via
  cosmic-config without restarting the panel
• A short click (< 400 ms) behaves exactly as before — popups open
  normally

Changes

• panel_space.rs: AppletDragState, DragSection, DRAG_LONG_PRESS_MS,
  find_applet_for_surface(), update_drag_preview(), commit_drag_reorder()

• wrapper_space.rs: long-press detection in update_pointer(),
  drag start/commit in handle_button(), press suppression via
  is_pending_drag() to prevent popups from opening during drag

• layout.rs: temporary reorder during drag preview + slide animation

• main.rs: PanelCalloopMsg::ReorderPlugins to persist order without
  triggering a panel restart

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