feat(design): the channel menu's Tags and Priority submenus (C1, E28 S3e3b)

Tags A-Z as in-place checks (the menu stays open for the next); Priority
as the fixed steps 10 5 3 2 1 0, an off-scale current value ticked in its
place. A pick re-sorts the list at once, moving only that row
(placeByPriority, the server's order), and the table / card pager and
the focus follow it (DataTable and useCardPager take a reveal seq,
listNav.focusNavItem lands on the row's own "…", a virtualized card
revealed first). ActionMenu rows take check: many | one. The menu reads
the row fresh from the cache and keys its target by id.

frontend/src/lib/listNav.ts

@@ -213,20 +213,44 @@ function focusItem(el: HTMLElement, reveal = true): void {
   if (reveal) el.scrollIntoView?.({ block: "nearest" });
 }
 
-function focusIndex(list: NavList, index: number): void {
+function focusIndex(
+  list: NavList,
+  index: number,
+  land: (el: HTMLElement) => void = (el) => focusItem(el),
+): void {
   const el = itemAt(list, index);
-  if (el) return focusItem(el);
+  if (el) return land(el);
   // Virtualized and not mounted: have the list scroll it in, then take it as soon as it renders.
   list.reveal(index);
   let frames = 0;
   const wait = () => {
     const got = itemAt(list, index);
-    if (got) focusItem(got);
+    if (got) land(got);
     else if (++frames < REVEAL_FRAMES) requestAnimationFrame(wait);
   };
   requestAnimationFrame(wait);
 }
 
+/** Hand the focus to item `index` of the list registered inside `within` — or, with `pick`, to the
+ *  control `pick` finds in it (the row's own "…") — bringing a virtualized item into the DOM first
+ *  and scrolling it just far enough to be seen. For a caller that MOVED an item and follows it: a
+ *  channel re-sorted by the priority its menu set, whose node may have been re-inserted (the focus
+ *  then fell to <body>) or scrolled off. */
+export function focusNavItem(
+  within: HTMLElement,
+  index: number,
+  pick?: (item: HTMLElement) => HTMLElement | null,
+): void {
+  const list = [...lists.values()].find((l) => within.contains(l.el));
+  if (!list) return;
+  focusIndex(list, index, (item) => {
+    const to = pick?.(item);
+    if (!to) return focusItem(item);
+    to.focus({ preventScroll: true });
+    item.scrollIntoView?.({ block: "nearest" });
+  });
+}
+
 function step(delta: 1 | -1): void {
   const cur = current();
   if (!cur) {