feat: ask which games you play on first run

A calendar full of games you don't play is worse than an empty one — it
buries the thing you came for.

Nothing is preselected; the button stays disabled until a game is picked,
which is clearer than guessing and hoping the reader notices. The choice is
stored as hiddenGames, the inverse, so a game added later shows up by
default rather than staying invisible.

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

src/client/state/usePrefs.ts

@@ -10,6 +10,8 @@ export interface Prefs {
   showCompleted: boolean;
   /** False until the reader confirms or changes the guessed region. */
   regionConfirmed: boolean;
+  /** False until the reader has picked their games on first run. */
+  onboarded: boolean;
 }
 
 function defaults(): Prefs {
@@ -18,6 +20,7 @@ function defaults(): Prefs {
     hiddenGames: [],
     showCompleted: true,
     regionConfirmed: false,
+    onboarded: false,
   };
 }