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/lib/channelLayout.ts

@@ -1,33 +1,30 @@
 // The channel manager's layout vocabulary (E4 S3). Named `ChannelLayout` — NOT `ChannelView` — on
 // purpose: the manager already has a `ChannelsView` (the subscribed/discovery/blocked TAB), and a
-// one-letter-apart type would be a footgun. This is the table-vs-card-vs-row density switch, the
-// sibling of the feed's `FeedView`; it rides the same generic `ViewSwitcher`.
-export type ChannelLayout = "table" | "cards" | "rows";
+// one-letter-apart type would be a footgun. This is the table-vs-card density switch, riding the
+// same generic `ViewSwitcher` as the feed. Two layouts: the DataTable, and a virtualized card grid.
+// (A `rows` variant existed briefly but was retired — it barely differed from the table.)
+export type ChannelLayout = "table" | "cards";
 
-export const CHANNEL_LAYOUTS: readonly ChannelLayout[] = ["table", "cards", "rows"] as const;
+export const CHANNEL_LAYOUTS: readonly ChannelLayout[] = ["table", "cards"] as const;
 
 const CHANNEL_LAYOUT_DEFAULT: ChannelLayout = "table";
 
-/** The card/row layouts' grid spec, consumed by VirtualGrid (S3.3). `table` has no entry — it keeps
- * the numbered-pager DataTable, not the virtualized grid. Tuned by measuring the real layouts. */
+/** The `cards` layout's grid spec, consumed by VirtualGrid. `table` has no entry — it keeps the
+ * numbered-pager DataTable, not the virtualized grid. */
 export interface ChannelLayoutSpec {
-  /** Narrowest grid column in px, or null for ONE full-width column (the row list). */
-  minCol: number | null;
-  /** Single-column reading-width cap in px (null = fill). */
-  maxCol: number | null;
-  /** First-paint row-height estimate; real heights are measured per-row after. */
+  /** Narrowest grid column in px. */
+  minCol: number;
+  /** First-paint row-height estimate; real heights are measured per-row after. Measured in the
+   * browser (channel heading + About line + meta + tags + actions), not guessed. */
   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: 176 },
-  rows: { minCol: null, maxCol: 1100, rowEst: 62 },
+  cards: { minCol: 320, rowEst: 232 },
 };
 
-/** Coerce a stored value to a layout; anything unknown (corrupt, a rolled-back future layout) falls
- * back to the default rather than rendering nothing. */
+/** Coerce a stored value to a layout; anything unknown (corrupt, a rolled-back future layout, or the
+ * retired "rows") falls back to the default rather than rendering nothing. */
 export function parseChannelLayout(raw: unknown): ChannelLayout {
   return (CHANNEL_LAYOUTS as readonly string[]).includes(raw as string)
     ? (raw as ChannelLayout)