feat(ui): PageUp/PageDown page through every dropdown list, not just the palette

The palette got PageUp/PageDown, and the same crawl stayed everywhere else a list opens under a
control: the sidebar's channel filter, the Plex genre/studio/collection pickers, the filter and sort
menus, submenus of every playlist you own. floating-ui's list navigation has only the arrows and
Home/End.

- `lib/listPaging`: the one definition of a page (the rows wholly in view minus one, a fixed 10 with
  no layout) and of where it lands (stops at both ends, never on a row it may not take, looking back
  toward the start rather than past it). The palette now uses it instead of its own copy, and
  `listNav` takes its scroll-parent helper from here.
- `AnchoredPopover`: one keydown handler on the reference AND the surface, so a combobox (keys on its
  input, `aria-activedescendant`) and a roving menu (keys on the focused row) both page. It runs after
  floating-ui's own, which has by then switched to keyboard modality and scrolls the new row in. A
  disabled row is never the target; a submenu's key does not also page the menu around it; with no
  mounted rows (closed, a hover card, a dialog) the key keeps its native meaning.

Measured in the browser: the channel filter shows 8 rows and pages by 7 with the active row in view
after the scroll; a menu pages to its last row and back, the page behind it does not scroll; the
palette still pages by 11 of its 12 visible rows.

frontend/src/lib/listNav.ts

@@ -1,5 +1,6 @@
 import { useEffect, useRef, type RefObject } from "react";
 import { dispatch, useKeyScope } from "./keymap";
+import { scrollParent } from "./listPaging";
 
 // ── Keyboard navigation over lists and grids (E11 S4, UX-6) ──────────────────────────────────────
 // Before this, a feed card cost ~10 Tab stops (thumb, play, info, title, channel, five actions), so
@@ -64,14 +65,6 @@ function current(): { list: NavList; index: number; item: HTMLElement } | null {
   return item && list ? { list, index: indexOf(item), item } : null;
 }
 
-function scrollParent(node: HTMLElement): HTMLElement | null {
-  for (let el = node.parentElement; el; el = el.parentElement) {
-    const oy = getComputedStyle(el).overflowY;
-    if (oy === "auto" || oy === "scroll") return el;
-  }
-  return null;
-}
-
 /** Where `j` lands when nothing in a list has focus yet: the first item the reader can see, not the
  *  top of the list — after scrolling down a feed, the first press must not throw them back up. "Can
  *  see" = its bottom edge is below the top of the scroller's visible band, which starts under the