fix(ui): a sticky table header turns the scroller's top fade off itself; DataTable renders one layout

- "A page with a sticky ColumnHead must declare fadeTop: false" lived in
  prose: a registry flag on three pages plus an App special case for the
  Channels card layout, and the next table page would have re-acquired the
  fade-on-the-header defect with every lane green. ColumnHead now registers
  itself with the enclosing PageScroller (ui/stickyTop) while mounted; the
  registry flag, the App branch and the prop plumbing are gone. Measured on
  localdev: the top mask is 0px under Channels' and Audit's headers, 20px on
  Home and the Feed.
- ui/DataTable mounted its table AND its card list at every width, one
  hidden by CSS: double DOM and render cost, duplicate ids, two keyboard
  copies of every row. It now mounts the one the `md` media query calls for
  (lib/useMediaQuery); the keyboard test asserts one layout at each width.
- The Plex module's content width went px → rem with the rest of the page.

frontend/src/App.tsx

@@ -20,7 +20,6 @@ import { usePref } from "./lib/usePref";
 import { useVerifyEntry } from "./lib/useVerifyEntry";
 import { useNavigation, useNavigationActions } from "./components/navigationContext";
 import { useFeedFilters, useFeedFiltersActions } from "./components/feedFiltersContext";
-import { useChannelLayout } from "./components/channelsContext";
 import { useFeedView, useFeedViewActions } from "./components/feedViewContext";
 import { moduleDef, moduleOrder } from "./lib/modules";
 import { PAGE_CONTENT, PAGE_RAIL, preloadPages } from "./components/moduleRegistry";
@@ -71,11 +70,6 @@ export default function App() {
   // handlers that mutate the feed view (onShowInFeed, tag-click, the feed scrollKey).
   const filters = useFeedFilters();
   const { setFilters } = useFeedFiltersActions();
-  // The channel manager turns its top fade OFF for the table (its sticky header sits at the scroller
-  // top) but wants it ON for the card layout (no sticky header) — so the fade depends on the layout.
-  // Deliberately the NARROW layout context: subscribing to the manager's whole state here would
-  // re-render the app root on every keystroke in its search box.
-  const channelLayout = useChannelLayout();
   // The feed's view mode (card/row/tile density) lives in FeedViewProvider — server-synced, shared
   // by the feed page and the channel page. App reads it only to hand to <Feed>/<ChannelPage>.
   const view = useFeedView();
@@ -325,7 +319,7 @@ export default function App() {
   const me = meQuery.data!;
   const pageGate = moduleDef(page).gate;
   const activePage: Page = !pageGate || pageGate(me) ? page : "feed";
-  const { Component: PageContent, fadeTop, maxWidth } = PAGE_CONTENT[activePage];
+  const { Component: PageContent, maxWidth } = PAGE_CONTENT[activePage];
   const railDef = channelView ? undefined : PAGE_RAIL[activePage];
 
   return (
@@ -392,9 +386,6 @@ export default function App() {
           <PageShell
             scrollKey={normalScrollKey}
             maxWidth={maxWidth}
-            // Channels: the registry disables the top fade for its sticky-header table, but the card
-            // layout has no sticky header, so re-enable it there.
-            fadeTop={activePage === "channels" ? channelLayout === "cards" : (fadeTop ?? true)}
             header={<Header me={meQuery.data!} onYtSearch={enterYtSearch} />}
             banners={
               <>