feat: file manager on SSH hosts through a machine - open from the plus menu

internal/server/web/static/app.css

@@ -757,5 +757,6 @@ p.mv-a { margin:0; font-size:13px; }
 /* The pane: its bar and list scroll inside it. */
 .fpane-body { flex:1; min-height:0; overflow:auto; padding:10px 12px 12px; }
 .tabview:not(.split) .fpane-body { padding:12px 16px 16px; }
-/* A lone file pane: its first bar keeps clear of the corner "+". */
-.tabview:not(.split) .fpane .fm-bar:first-child, body.split1 .fpane .fm-bar:first-child { margin-right:40px; }
+/* The file pane under the corner "+" keeps its first bar clear of it,
+   while the "+" is there (one tab; with the tab strip it is hidden). */
+body:not(.multi) .fpane.under-plus .fm-bar:first-child { margin-right:40px; }