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/moduleRegistry.tsx

@@ -39,9 +39,6 @@ export type PageDef = {
   // Prop-free by construction: reads `me` + module state from hooks/providers. Preloadable by type, so
   // a new page cannot silently opt out of preloadPages.
   Component: PreloadableComponent<object>;
-  // Channels + Audit render a table with a sticky header at the scroller top; the shell's top fade
-  // would sit on it and flip on with the first scroll, so they turn it off. Defaults to on.
-  fadeTop?: boolean;
   // The content column's max width. The header + toolbar centre on this same width (via --content-mw)
   // so the floating chrome lines up with the centred content instead of spanning the whole column.
   // MUST match the max-w the page's own content wrapper uses. Omit for a full-width page (feed).
@@ -53,9 +50,9 @@ export const PAGE_CONTENT: Record<Page, PageDef> = {
   home: { Component: Home, maxWidth: "80rem" },
   // The two wrappers preload the lazy chunk they render.
   feed: { Component: Object.assign(FeedPage, { preload: Feed.preload }) },
-  channels: { Component: Channels, fadeTop: false, maxWidth: "96rem" },
-  playlists: { Component: Playlists, fadeTop: false, maxWidth: "64rem" },
-  plex: { Component: PlexBrowse, maxWidth: "1600px" },
+  channels: { Component: Channels, maxWidth: "96rem" },
+  playlists: { Component: Playlists, maxWidth: "64rem" },
+  plex: { Component: PlexBrowse, maxWidth: "100rem" },
   notifications: { Component: NotificationsPanel, maxWidth: "48rem" },
   messages: { Component: Object.assign(MessagesPage, { preload: Messages.preload }) },
   downloads: { Component: DownloadCenter, maxWidth: "56rem" },
@@ -63,7 +60,7 @@ export const PAGE_CONTENT: Record<Page, PageDef> = {
   scheduler: { Component: Scheduler },
   config: { Component: ConfigPanel },
   users: { Component: AdminUsers, maxWidth: "48rem" },
-  audit: { Component: AuditLog, fadeTop: false },
+  audit: { Component: AuditLog },
   logs: { Component: AdminLogs },
   settings: { Component: SettingsPanel },
 };