Excel-style column filters, full-page Custom View, card field editor

Filters (web/excel-filter.js), modeled on Excel's AutoFilter:
- Every column header gets a dropdown: Sort A to Z / Z to A, Clear Filter
  From "<column>", Search, a checkbox list of the column's values with
  (Select All) and (Blanks), OK / Cancel. Values come from rows passing
  the other columns' filters; filters on several columns combine. Tags
  and software lists are split into individual items.
- The button shows a funnel when filtered and an arrow when sorted, and a
  "Clear filters (n)" button appears beside the group's copy buttons
- Replaces the old per-column filter row (which also removes the gap
  between header and filter rows); hooks the existing filter passes so
  global search, counts and worker re-renders keep working
- Menus are fixed-position on <body>, so sticky headers and neighbouring
  tables can't cover them; on phones they open as a bottom sheet
- Workbench stacks get the same menus (sorting done client-side)
- Custom View's filter icon opens the same menu

Layout:
- Sticky headers use separated borders so no hairline gap opens
- Phones/tablets: a sticky bar of column chips (with active counts and
  "Clear all") opens the filter menus
- Custom View fills the page like the other views on desktop; it still
  scrolls sideways when wide, with its header pinned under the group header
- Custom View on phones/tablets: a "Card fields" sheet to choose, order
  and remove the fields shown on each card

Demo: first visit puts 6 PCs (one per migration stage) on the Workbench.

web/index.html

@@ -353,6 +353,7 @@
     </style>
     <link rel="stylesheet" href="/layout.css">
     <script src="/layout.js"></script>
+    <script src="/excel-filter.js" defer></script>
 </head>
 <body>