refactor(managers): full About overlay + center the wider channel content (E4 S2 follow-up)

- The About overlay now shows the FULL channel About (description, external links,
  country, language, topics, keywords) — the same content as the detail page's About
  tab, via a new shared ChannelAboutContent component (reused by ChannelPage + AboutCell)
- AboutCell fetches the channel detail ON DEMAND on hover (shared [channel,id] react-query
  cache with the channel page), so the list stays light; the overlay is interactive
  (scrollable + clickable links) via a small hover-bridge, and closes on page scroll/resize
- Center the content: max-w-7xl -> max-w-[96rem] on the channel tables + header, so the
  wider About table stays symmetric with the header (mx-auto handles the responsive centering)
- Topic chips are display-only for now; the future search epic makes them clickable

frontend/src/components/ChannelDiscovery.tsx

@@ -99,7 +99,7 @@ export default function ChannelDiscovery({
       key: "about",
       header: t("channels.cols.about"),
       hideInCard: true,
-      render: (c) => <AboutCell text={c.description} />,
+      render: (c) => <AboutCell text={c.description} channelId={c.id} />,
     },
     subsColumn<DiscoveredChannel>(t),
     {
@@ -165,11 +165,11 @@ export default function ChannelDiscovery({
     <>
       {/* Intro + the table controls are fixed chrome; only the table rows scroll (sticky header). */}
       <PageToolbar>
-        <div className="px-4 pt-3 max-w-7xl mx-auto">
+        <div className="px-4 pt-3 max-w-[96rem] mx-auto">
           <p className="text-xs text-muted leading-relaxed">{t("channels.discovery.intro")}</p>
         </div>
       </PageToolbar>
-      <div className="px-4 pb-4 max-w-7xl mx-auto">
+      <div className="px-4 pb-4 max-w-[96rem] mx-auto">
         {query.isLoading ? (
           <div className="text-muted py-8">{t("channels.discovery.loading")}</div>
         ) : (
@@ -180,7 +180,7 @@ export default function ChannelDiscovery({
             persistKey={accountKey(LS.channelDiscoveryTable) ?? undefined}
             controlsPosition="both"
             controlsInBand
-            controlsBandClassName="px-4 max-w-7xl mx-auto"
+            controlsBandClassName="px-4 max-w-[96rem] mx-auto"
             emptyText={t("channels.discovery.empty")}
           />
         )}