fix(tray): stop reorderItems from re-adding items (iterator UAF + double add)

The item-ordering feature made Host::reorderItems() re-run the full
remove/add path over items_ via std::ranges::for_each(on_remove_/on_add_).
This caused two confirmed bugs:

BUG 1 (iterator invalidation / UAF): on_add_ (Tray::onAdd) calls
Host::checkIgnoreList, which erases from items_ while for_each is still
iterating items_, invalidating iterators/pointers. Triggered by a
non-empty ignore-list matching an item with >=2 items present.

BUG 2 (double add): reorderItems runs while an item's Id is resolved in
proxyReady, i.e. before setReady(). It added the not-yet-ready item
(re-parenting its event_box, pushing into Tray::items_, connecting
signal_show/hide), then setReady() -> itemReady -> onAdd added it again:
GTK 'widget already has a parent' critical, duplicate Item* and
signal handlers that accumulated unbounded.

Fixes:
- reorderItems() now only reorders already-added GTK box children via a
  dedicated on_reorder_ callback (Tray::reorderBox), never re-adding or
  removing. reorderBox stable-sorts items_ by order_ and repositions
  children with gtk_box_reorder_child (honouring reverse-direction).
- Tray::onAdd is idempotent (guards against an already-added item) and
  positions the new widget via reorderBox before the ignore-list check.
- signal_show/signal_hide connections are stored per item and
  disconnected in Tray::onRemove; onRemove is a no-op for items that were
  never added.

include/modules/sni/host.hpp

@@ -16,7 +16,8 @@ class Host {
  public:
   Host(std::size_t id, const Json::Value&, const Bar&, const std::vector<std::string>&,
        const std::function<void(std::unique_ptr<Item>&)>&,
-       const std::function<void(std::unique_ptr<Item>&)>&, const std::function<void()>&);
+       const std::function<void(std::unique_ptr<Item>&)>&, const std::function<void()>&,
+       const std::function<void()>&);
   ~Host();
 
   void checkIgnoreList(const std::vector<std::string>& ignore_list,
@@ -55,6 +56,10 @@ class Host {
   const std::vector<std::string> ignore_list_;
   const std::function<void(std::unique_ptr<Item>&)> on_add_;
   const std::function<void(std::unique_ptr<Item>&)> on_remove_;
+  // Re-applies the configured ordering to the already-added tray widgets. This
+  // must NOT re-run the add path (which would re-parent widgets and reconnect
+  // signals); it only reorders existing children.
+  const std::function<void()> on_reorder_;
 
   ItemOrderMap orders_;
   const std::function<void()> on_update_;