feat(managers): actions-first discovery table, tabs in the back chain, pager scrolls to top

UAT round 4:
- Discovery table: move Actions to the first column, mirroring the Subscriptions
  table (where it sits right after Prio) so both tabs' actions line up.
- The manager's tabs now ride in history.state (`_chTab`), the same way the channel
  page (`_chan`) and the live search (`_yt`) already do — so Back steps back through
  the tabs you visited before it leaves the module. Arriving from elsewhere stamps
  the entry setPage pushed (one navigation never costs two Back presses).
- Turning a page now scrolls the list back to its top (both the tables and the card
  grid, since they share Pager) — landing mid-list in an unseen page is disorienting.

frontend/src/components/ChannelDiscovery.tsx

@@ -97,6 +97,34 @@ export default function ChannelDiscovery({
   );
 
   const columns: Column<DiscoveredChannel>[] = [
+    // Actions first, mirroring the Subscriptions table (where it sits right after Prio) so the two
+    // tabs' action clusters line up in the same place.
+    {
+      key: "actions",
+      header: t("channels.cols.actions"),
+      align: "left",
+      nowrap: true,
+      width: "120px",
+      cardLabel: false,
+      render: (c) => (
+        <Tooltip
+          hint={
+            canWrite
+              ? t("channels.discovery.subscribeHint")
+              : t("channels.discovery.needWriteHint")
+          }
+        >
+          <button
+            onClick={() => onSubscribe(c)}
+            disabled={!canWrite || subscribe.isPending}
+            className="glass-card glass-hover inline-flex items-center gap-1.5 px-2.5 py-1.5 rounded-lg text-xs disabled:opacity-50 transition"
+          >
+            <UserPlus className="w-3.5 h-3.5" />
+            {t("channels.discovery.subscribe")}
+          </button>
+        </Tooltip>
+      ),
+    },
     {
       key: "channel",
       header: t("channels.cols.channel"),
@@ -156,32 +184,6 @@ export default function ChannelDiscovery({
         </Tooltip>
       ),
     },
-    {
-      key: "actions",
-      header: t("channels.cols.actions"),
-      align: "right",
-      nowrap: true,
-      width: "120px",
-      cardLabel: false,
-      render: (c) => (
-        <Tooltip
-          hint={
-            canWrite
-              ? t("channels.discovery.subscribeHint")
-              : t("channels.discovery.needWriteHint")
-          }
-        >
-          <button
-            onClick={() => onSubscribe(c)}
-            disabled={!canWrite || subscribe.isPending}
-            className="glass-card glass-hover inline-flex items-center gap-1.5 px-2.5 py-1.5 rounded-lg text-xs disabled:opacity-50 transition"
-          >
-            <UserPlus className="w-3.5 h-3.5" />
-            {t("channels.discovery.subscribe")}
-          </button>
-        </Tooltip>
-      ),
-    },
   ];
 
   // Card layout paginates like the table (slice the sorted rows; the Pager reports changes).