Fix /logout hang caused by blocking worker thread joins

unloadAll() joins worker threads which blocks if they're mid-tile
(prepareTile can take seconds for heavy ADTs). Replace with softReset()
which clears tile data, queues, and water surfaces without stopping
worker threads — workers find empty queues and idle naturally.

include/rendering/terrain_manager.hpp

@@ -212,6 +212,7 @@ public:
      * Unload all tiles
      */
     void unloadAll();
+    void softReset();  // Clear tile data without stopping worker threads (non-blocking)
 
     /**
      * Precache a set of tiles (for taxi routes, etc.)