physics: implement player-controlled flying mount physics

When CAN_FLY + FLYING movement flags are both set (flying mounts, Druid
Flight Form), the CameraController now uses 3D pitch-following movement
instead of ground physics:
- Forward/back follows the camera's 3D look direction (ascend when
  looking up, descend when looking down)
- Space = ascend vertically, X (while mounted) = descend
- No gravity, no grounding, no jump coyote time
- Fall-damage checks suppressed (grounded=true)

Also wire up all remaining server movement state flags to CameraController:
- Feather Fall: cap terminal velocity at -2 m/s
- Water Walk: clamp to water surface, skip swim entry
- Flying: 3D movement with no gravity

All states synced each frame from GameHandler via isPlayerFlying(),
isFeatherFalling(), isWaterWalking(), isGravityDisabled().

include/rendering/camera_controller.hpp

@@ -99,6 +99,7 @@ public:
     void setGravityDisabled(bool disabled) { gravityDisabled_ = disabled; }
     void setFeatherFallActive(bool active) { featherFallActive_ = active; }
     void setWaterWalkActive(bool active) { waterWalkActive_ = active; }
+    void setFlyingActive(bool active) { flyingActive_ = active; }
     void setMounted(bool m) { mounted_ = m; }
     void setMountHeightOffset(float offset) { mountHeightOffset_ = offset; }
     void setExternalFollow(bool enabled) { externalFollow_ = enabled; }
@@ -285,6 +286,8 @@ private:
     bool featherFallActive_ = false;
     // Server-driven water walk: treat water surface as ground (don't swim).
     bool waterWalkActive_ = false;
+    // Player-controlled flight (CAN_FLY + FLYING): 3D movement, no gravity.
+    bool flyingActive_ = false;
     bool mounted_ = false;
     float mountHeightOffset_ = 0.0f;
     bool externalMoving_ = false;