feat(gameplay): add walking sound for players and adjust underwater bubble interval

- Move walk/run sound interval constants from anonymous namespace to ClientPlayer header
- Add moving_time field to PlayerInfo to track continuous movement
- Implement play_walk_sound callback triggered by WALK and RUN gaits
- Increase underwater bubble timer from 1.0s to 1.5s

include/Cubed/gameplay/client_player.hpp

@@ -17,6 +17,8 @@ namespace Cubed {
 class ClientWorld;
 class ClientPlayer {
 public:
+    static constexpr float WALK_SOUND_INTERVAL = 0.45f;
+    static constexpr float RUN_SOUND_INTERVAL = 0.3f;
     using ChunkPosSet = absl::flat_hash_set<ChunkPos, ChunkPos::Hash>;
     ClientPlayer(ClientWorld& world);
     ~ClientPlayer();

include/Cubed/gameplay/client_world.hpp

@@ -28,6 +28,7 @@ struct PlayerInfo {
     Gait gait;
     float angle = 0.0f;
     float walk_time = 0.0f;
+    float moving_time = 0.0f;
 };
 
 struct PlayerRenderData {