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/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