Sticky status headers; fix floating details-view button on phones

- Status group headers ("Not Started (37)") stick under the top nav while
  scrolling through their group, on every screen size
- On desktop the column header and filter rows stick beneath the group
  header (table wrappers no longer clip them; these fixed-layout tables
  never scroll sideways at desktop widths)
- layout.js measures the nav, group header and header row heights so the
  sticky rows stack without gaps, re-measuring on re-render/resize/scroll
- Phones: the Advanced/Simple View button sits on its own line under the
  device name instead of floating over the dialog content
- Rename mobile.css/js to layout.css/js now that they hold desktop rules

web/index.html

@@ -351,8 +351,8 @@
             .status-btn:hover { color: white; background: rgba(255,255,255,0.1); }
         }
     </style>
-    <link rel="stylesheet" href="/mobile.css">
-    <script src="/mobile.js"></script>
+    <link rel="stylesheet" href="/layout.css">
+    <script src="/layout.js"></script>
 </head>
 <body>