feat: let readers ignore events they don't care about

Ignoring is not completing. "Done" keeps an event visible and counted;
"not interested" removes it from both views, which is the entire point.

Completions and ignores are the same shape and want the same guarantees, so
they now share one mark-set implementation — union merge on import, never a
removal, since nothing else holds a copy. Both ride in the export file.

Ignored events stay recoverable: the count and a reveal toggle appear in
settings once there is something to reveal.

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

src/client/state/usePrefs.ts

@@ -8,6 +8,8 @@ export interface Prefs {
   /** Games the reader has switched off. Stored as hidden so a newly added game shows up by default. */
   hiddenGames: GameId[];
   showCompleted: boolean;
+  /** Reveal events the reader has ignored, so they can be restored. */
+  showIgnored: boolean;
   /** False until the reader confirms or changes the guessed region. */
   regionConfirmed: boolean;
   /** False until the reader has picked their games on first run. */
@@ -19,6 +21,7 @@ function defaults(): Prefs {
     region: guessRegion(),
     hiddenGames: [],
     showCompleted: true,
+    showIgnored: false,
     regionConfirmed: false,
     onboarded: false,
   };