UX: WebView native-feel + Context-Menu Redesign + Startup-Fix

- Disable text selection (CSS user-select:none) + block browser
  context menu (contextmenu preventDefault) in all panels
- ContextMenu: pill items, accent hover, entrance animation,
  optional title header — controlled via single ContextMenu.css
- EbenenManager + GeschossManager: show layer name as menu title
- startup.py: skip splash on Cmd+N when plugin already loaded
- startup.py: hook NewDocument so display-modes apply to new docs

rhino/panel_base.py

@@ -267,10 +267,12 @@ def _build_inline_template():
         html = f.read().decode("utf-8")
 
     placeholder_script = '<script>' + _MODE_SCRIPT_PLACEHOLDER + '</script>'
+    _no_select = '<style>*{-webkit-user-select:none!important;user-select:none!important;}</style>'
+    _no_ctx    = '<script>document.addEventListener("contextmenu",function(e){e.preventDefault();},true);</script>'
     if "</head>" in html:
-        html = html.replace("</head>", placeholder_script + "</head>")
+        html = html.replace("</head>", placeholder_script + _no_select + _no_ctx + "</head>")
     else:
-        html = placeholder_script + html
+        html = placeholder_script + _no_select + _no_ctx + html
 
     def inline_css(m):
         p = os.path.join(dist_dir, m.group(1).lstrip("./").replace("/", os.sep))
@@ -353,6 +355,15 @@ def attach_webview(panel, bridge, mode):
             wv.ExecuteScript("window.RHINO_MODE=true;")
         except Exception:
             pass
+        try:
+            wv.ExecuteScript(
+                "var _ds=document.createElement('style');"
+                "_ds.textContent='*{-webkit-user-select:none!important;user-select:none!important;}';"
+                "document.head.appendChild(_ds);"
+                "document.addEventListener('contextmenu',function(e){e.preventDefault();},true);"
+            )
+        except Exception:
+            pass
 
     def on_idle(s, e):
         Rhino.RhinoApp.Idle -= on_idle
@@ -440,6 +451,14 @@ def open_satellite_window(mode, params=None, title=None, size=(420, 560),
     def on_loaded(s, e):
         try: wv.ExecuteScript("window.RHINO_MODE=true;")
         except Exception: pass
+        try:
+            wv.ExecuteScript(
+                "var _ds=document.createElement('style');"
+                "_ds.textContent='*{-webkit-user-select:none!important;user-select:none!important;}';"
+                "document.head.appendChild(_ds);"
+                "document.addEventListener('contextmenu',function(e){e.preventDefault();},true);"
+            )
+        except Exception: pass
 
     wv.DocumentTitleChanged += on_title_
     wv.DocumentLoaded       += on_loaded