feat(managers): richer channel cards + drop the rows layout, cap the channel column

UAT round 1 feedback:
- Retire the `rows` layout — it barely differed from the table; keep Table + Cards.
- The card now shows the About cell (+ its hover overlay) and the priority stepper
  in every layout, not just the table: ChannelLayoutGrid gains named slots
  (lead / about / actions) and stops hiding the `hideInCard` columns, since the
  roomy desktop card has space the cramped mobile fallback did not.
- Fix the table losing its centering with "Rows per page: All": a long-titled
  channel stretched the uncapped Channel column (~289→429px) and overflowed the
  centered table. Cap it with a max-w element inside the cell (table-layout:auto
  ignores a cell max-width), the same trick AboutCell already uses. Gaps now 163/162.

frontend/src/components/ChannelDiscovery.tsx

@@ -89,14 +89,18 @@ export default function ChannelDiscovery({
       sortValue: (c) => (c.title ?? c.id).toLowerCase(),
       filter: { kind: "text", get: (c) => `${c.title ?? ""} ${c.handle ?? ""}` },
       cardPrimary: true,
+      // Cap the width (a max-w element inside the cell; table-layout:auto ignores a cell max-width) so
+      // a long-titled channel can't stretch the column and overflow the centered table.
       render: (c) => (
-        <ChannelLink
-          id={c.id}
-          title={c.title}
-          handle={c.handle}
-          thumbnailUrl={c.thumbnail_url}
-          onView={() => onViewChannel(c.id, c.title ?? c.id)}
-        />
+        <div className="max-w-[18rem]">
+          <ChannelLink
+            id={c.id}
+            title={c.title}
+            handle={c.handle}
+            thumbnailUrl={c.thumbnail_url}
+            onView={() => onViewChannel(c.id, c.title ?? c.id)}
+          />
+        </div>
       ),
     },
     {
@@ -192,7 +196,7 @@ export default function ChannelDiscovery({
             rows={rows}
             columns={columns}
             rowKey={(c) => c.id}
-            layout={layout}
+            aboutKey="about"
             actionsKey="actions"
             emptyText={t("channels.discovery.empty")}
           />