refactor(feed): cap the read-ahead lookahead and extract it to a tested helper

Review follow-up. The viewport-scaled lookahead was unbounded, so a tall window
prefetched several feed pages on every mount / filter change before any scroll —
wasted work on the app's heaviest query. Clamp it to MAX_LOOKAHEAD_ROWS, and move
the policy into a pure readAhead.ts (readAheadLookahead / shouldPrefetchNext) with
unit tests covering the floor, the scaling, the cap boundary, and the fetch trigger
— the core of the feed-scroll fix now has coverage instead of only manual E2E.

frontend/src/components/readAhead.ts

@@ -0,0 +1,30 @@
+// Infinite-scroll read-ahead policy for VirtualGrid, extracted as a pure function so the decision is
+// unit-testable without a layout engine.
+//
+// The next page is pulled once the last rendered row comes within `lookahead` rows of the loaded end,
+// where lookahead scales with the number of rows the viewport currently renders (so it adapts to
+// window height / column count / row height) but is clamped to a floor and a ceiling:
+//   - floor PREFETCH_ROWS: the finite-list / first-paint case where nothing is rendered yet.
+//   - ceiling MAX_LOOKAHEAD_ROWS: a tall window renders many rows, and an uncapped lookahead would
+//     eagerly prefetch several pages on every mount / filter change — wasted work on the app's
+//     heaviest query (the feed). The cap bounds that while still buffering a couple of screens ahead.
+export const PREFETCH_ROWS = 4;
+export const PREFETCH_VIEWPORTS = 2;
+export const MAX_LOOKAHEAD_ROWS = 40;
+
+/** How many rows ahead of the last rendered row to keep buffered, for a viewport rendering
+ *  `renderedRows` rows. Clamped to [PREFETCH_ROWS, MAX_LOOKAHEAD_ROWS]. */
+export function readAheadLookahead(renderedRows: number): number {
+  return Math.min(MAX_LOOKAHEAD_ROWS, Math.max(PREFETCH_ROWS, renderedRows * PREFETCH_VIEWPORTS));
+}
+
+/** Whether to fetch the next page: true once the last rendered row (`lastIndex`) is within the
+ *  read-ahead window of the loaded end (`rowCount` total loaded rows). Because the caller re-runs
+ *  this as each page settles, it chains — refilling the buffer as the user consumes it. */
+export function shouldPrefetchNext(
+  lastIndex: number,
+  rowCount: number,
+  renderedRows: number,
+): boolean {
+  return lastIndex >= rowCount - 1 - readAheadLookahead(renderedRows);
+}