feat(filter): focus one game at a time

Switching games on and off says which games you play, and is set once.
It's the wrong tool for what a player of four games does while reading —
clear one game, move to the next — which cost two taps per game and left
the settings panel no longer describing what they play.

Focus is a lens over that filter, not a second filter: a bar at the top,
above everything it narrows, with a "next game" control that steps
through and ends by returning to all. It never touches hiddenGames, and
a focus on a game since switched off is ignored rather than obeyed, so
it can't strand you on a blank page whose cause is elsewhere. Each chip
carries that game's outstanding count, so a game with nothing waiting
says so before you visit it.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/client/state/lens.ts

@@ -62,3 +62,46 @@ export function firstToExpire<T extends Row>(rows: readonly T[]): T | null {
   }
   return best ?? rows[0] ?? null;
 }
+
+/**
+ * The focused game, if it is still a game the reader can see.
+ *
+ * A focus on a game they have since switched off, or that has dropped out of
+ * the feed, is ignored rather than obeyed — the alternative is an empty page
+ * whose reason is a setting two screens away.
+ */
+export function resolveFocus(
+  focus: GameId | null,
+  enabled: readonly GameId[],
+): GameId | null {
+  return focus !== null && enabled.includes(focus) ? focus : null;
+}
+
+/**
+ * The next game in the rotation: all → first → … → last → all.
+ *
+ * Working through games one at a time is a loop, and it ends by coming back to
+ * everything rather than silently starting over — otherwise there is no way out
+ * of the rotation except finding the "all" chip again.
+ */
+export function advanceFocus(
+  focus: GameId | null,
+  enabled: readonly GameId[],
+): GameId | null {
+  if (enabled.length === 0) return null;
+  const at = focus === null ? -1 : enabled.indexOf(focus);
+  // An unknown focus (switched-off game) restarts the rotation rather than
+  // jumping to index 0 of nowhere.
+  return enabled[at + 1] ?? null;
+}
+
+/** How many rows each game still has outstanding, for the focus chips. */
+export function countByGame<T extends Row>(
+  rows: readonly T[],
+): Partial<Record<GameId, number>> {
+  const out: Partial<Record<GameId, number>> = {};
+  for (const row of rows) {
+    out[row.event.game] = (out[row.event.game] ?? 0) + 1;
+  }
+  return out;
+}

src/client/state/usePrefs.ts

@@ -8,6 +8,15 @@ export interface Prefs {
   region: Region;
   /** Games the reader has switched off. Stored as hidden so a newly added game shows up by default. */
   hiddenGames: GameId[];
+  /**
+   * One game to look at right now, or null for all of them.
+   *
+   * A lens, not a setting: it never changes `hiddenGames`, and a focus on a
+   * game that is switched off or has left the feed is ignored rather than
+   * obeyed (`resolveFocus`), so it can never leave the reader on a blank page
+   * with no visible cause.
+   */
+  focusGame: GameId | null;
   /** How the list is ordered. Deadline order is the default and the fallback. */
   sort: SortMode;
   /**
@@ -29,6 +38,7 @@ function defaults(): Prefs {
   return {
     region: guessRegion(),
     hiddenGames: [],
+    focusGame: null,
     sort: "ending",
     detectDaily: true,
     showCompleted: true,