fix(managers): key the row-layout primary cell + measured rowEst

Self-review follow-ups: the `rows` layout mapped the primary column without a
React key (single-element today, but a warning waiting to happen); wrap it in a
keyed Fragment. Also replace the guessed card/row rowEst with browser-measured
values (176 / 62) so the scrollbar doesn't jump on first scroll.

frontend/src/components/ChannelLayoutGrid.tsx

@@ -1,3 +1,4 @@
+import { Fragment } from "react";
 import { useTranslation } from "react-i18next";
 import { type Column } from "./DataTable";
 import VirtualGrid from "./VirtualGrid";
@@ -68,7 +69,11 @@ export default function ChannelLayoutGrid<T>({
 
   const renderRow = (row: T) => (
     <div className={`glass-card rounded-xl p-3 flex items-center gap-4 ${rowClassName?.(row) ?? ""}`}>
-      <div className="shrink-0 font-medium">{primary.map((col) => col.render(row))}</div>
+      <div className="shrink-0 font-medium">
+        {primary.map((col) => (
+          <Fragment key={col.key}>{col.render(row)}</Fragment>
+        ))}
+      </div>
       {metaFlow(row)}
       {actions && <div className="ml-auto shrink-0">{actions.render(row)}</div>}
     </div>

frontend/src/lib/channelLayout.ts

@@ -19,9 +19,11 @@ export interface ChannelLayoutSpec {
   rowEst: number;
 }
 
+// rowEst measured in the browser (cards ~176 at 3 columns, the one-line row ~62) — not guessed; a
+// bad estimate makes the scrollbar jump as you first scroll in.
 export const CHANNEL_LAYOUT_SPEC: Record<Exclude<ChannelLayout, "table">, ChannelLayoutSpec> = {
-  cards: { minCol: 300, maxCol: null, rowEst: 210 },
-  rows: { minCol: null, maxCol: 1100, rowEst: 88 },
+  cards: { minCol: 300, maxCol: null, rowEst: 176 },
+  rows: { minCol: null, maxCol: 1100, rowEst: 62 },
 };
 
 /** Coerce a stored value to a layout; anything unknown (corrupt, a rolled-back future layout) falls