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().

src/core/application.cpp

@@ -1015,6 +1015,7 @@ void Application::update(float deltaTime) {
                     renderer->getCameraController()->setGravityDisabled(gameHandler->isGravityDisabled());
                     renderer->getCameraController()->setFeatherFallActive(gameHandler->isFeatherFalling());
                     renderer->getCameraController()->setWaterWalkActive(gameHandler->isWaterWalking());
+                    renderer->getCameraController()->setFlyingActive(gameHandler->isPlayerFlying());
                 }
 
                 bool onTaxi = gameHandler &&