refactor: remove visual-servo feature

The marker-relative servo (endpoints /api/grasp/visual_servo/{plan,step,
plane/*}, helpers in grasp_routes.py, the bulk of grasp/visual_servo.py,
web/servo.js, and the Servo Visuale GUI tab) is gone. Plane fitting now
lives in /api/work_plane/* and perception/work_plane.py; Task 5 wires the
new Calibrazione Piano tab.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/presa_mano/main.py

@@ -103,7 +103,7 @@ def index() -> HTMLResponse:
     idx = WEB_DIR / "index.html"
     html = idx.read_text(encoding="utf-8")
     # Cache-bust app.js and style.css with their mtime so a rebuild forces refetch.
-    for name in ("app.js", "servo.js", "style.css"):
+    for name in ("app.js", "work_plane.js", "style.css"):
         f = WEB_DIR / name
         if f.exists():
             v = int(f.stat().st_mtime)