fix(ui): smooth B-spline section/waterline curves + 3D ready-signal sync

- viewer_3d.py: add `ready` Signal emitted once QtInteractor finishes init
- main_window.py: connect ready signal to sync active hull into 3D viewer on startup
- viewer_lines.py: add _smooth_pts helper; replace straight polylines in BodyPlanViewer
  and PlanViewer CAPA 3 with B-spline interpolated curves (80 sample points)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

arshipdesign/ui/widgets/viewer_3d.py

@@ -17,7 +17,7 @@ import logging
 from typing import Optional
 
 import numpy as np
-from PySide6.QtCore import Qt, QTimer
+from PySide6.QtCore import Qt, QTimer, Signal
 from PySide6.QtWidgets import QLabel, QVBoxLayout, QWidget
 
 logger = logging.getLogger("ui.viewer_3d")
@@ -50,6 +50,8 @@ class Viewer3DWidget(QWidget):
     >>> v.reset_camera()
     """
 
+    ready: Signal = Signal()
+
     def __init__(self, parent: Optional[QWidget] = None) -> None:
         super().__init__(parent)
         self._plotter: Optional["QtInteractor"] = None
@@ -114,6 +116,7 @@ class Viewer3DWidget(QWidget):
             else:
                 self._load_default_wigley()
             self._ready = True
+            self.ready.emit()
             logger.info("Viewer3DWidget: QtInteractor iniciado correctamente")
 
         except Exception as exc: