Fix underground spawns in Stormwind, redesign delete character button

Spawn fix: floor probe used terrain height (underground) instead of server
position when searching for spawn floor. Now probes from max(terrain, serverZ)
so WMO city surfaces above terrain are found correctly. Also invalidates
floor cache on respawn.

Delete button: moved from details panel to bottom row (small, red, far right).
Two-stage confirmation with modal popups: first asks "are you sure", second
warns "THIS CANNOT BE UNDONE" with red DELETE PERMANENTLY button.

include/ui/character_screen.hpp

@@ -64,7 +64,7 @@ private:
     std::function<void()> onCreateCharacter;
     std::function<void()> onBack;
     std::function<void(uint64_t)> onDeleteCharacter;
-    bool confirmDelete = false;
+    int deleteConfirmStage = 0;  // 0=none, 1=first warning, 2=final warning
 
     /**
      * Get faction color based on race