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/components/ui/PageShell.tsx

@@ -45,7 +45,6 @@ export default function PageShell({
   banners,
   children,
   scrollKey,
-  fadeTop = true,
   maxWidth,
 }: {
   header?: ReactNode;
@@ -53,8 +52,6 @@ export default function PageShell({
   children: ReactNode;
   // Identifies the logical view for the scroll restore/reset policy — see PageScroller.
   scrollKey?: string;
-  // Off when this page's scroll region starts with a sticky header — see PageScroller.
-  fadeTop?: boolean;
   // The content column's max width (e.g. "1600px"); the header + toolbar centre on the same width via
   // --content-mw. Omit for a full-width page (feed). See index.css --content-mw.
   maxWidth?: string;
@@ -145,9 +142,7 @@ export default function PageShell({
         {/* pr-[--sbw]: the toolbar band isn't the scroller, so without this it would run the
             scrollbar-gutter's width past the scrolled cards on the right. Pad it back to card width. */}
         <div ref={setToolbarSlot} className="shrink-0 pr-[var(--sbw)]" />
-        <PageScroller scrollKey={scrollKey} fadeTop={fadeTop}>
-          {children}
-        </PageScroller>
+        <PageScroller scrollKey={scrollKey}>{children}</PageScroller>
       </div>
     </PageToolbarSlot.Provider>
   );