Scope the leaderboard to a selectable game

Elo ratings are stored per game type, so the leaderboard now always
requests /api/leaderboard?game_type=<id> and shows only that game's
players, Elo, wins, matches and points instead of mixing all games'
aggregates with the default game's rating.

- api::leaderboard takes the game type;
- a "Game" dropdown next to the heading lists every registered game and
  reloads the board on change (stale in-flight responses are dropped);
- move fallback_game_types() from the lobby to model.rs so both the
  lobby and leaderboard selectors work when /api/game-types fails.

web/src/model.rs

@@ -408,6 +408,38 @@ pub struct GameTypesPage {
     pub results: Vec<GameTypeInfo>,
 }
 
+/// The games to show in the lobby's and leaderboard's game selectors when
+/// the `/api/game-types` fetch fails: match creation and per-game views
+/// must still work.
+pub fn fallback_game_types() -> Vec<GameTypeInfo> {
+    vec![
+        GameTypeInfo {
+            id: "scopone_scientifico".to_string(),
+            name: "Scopone scientifico".to_string(),
+            description: String::new(),
+            min_players: 4,
+            max_players: 4,
+            options_schema: serde_json::Value::Null,
+        },
+        GameTypeInfo {
+            id: "briscola".to_string(),
+            name: "Briscola".to_string(),
+            description: String::new(),
+            min_players: 2,
+            max_players: 2,
+            options_schema: serde_json::Value::Null,
+        },
+        GameTypeInfo {
+            id: "briscola_a_quattro".to_string(),
+            name: "Briscola a quattro".to_string(),
+            description: String::new(),
+            min_players: 4,
+            max_players: 4,
+            options_schema: serde_json::Value::Null,
+        },
+    ]
+}
+
 /// One configured OIDC login provider (GET /api/auth/providers).
 #[derive(Debug, Clone, PartialEq, Deserialize)]
 pub struct AuthProvider {
@@ -1142,6 +1174,18 @@ mod tests {
         assert_eq!("?", suit_name("?"));
     }
 
+    #[test]
+    fn fallback_game_types_cover_the_registered_games() {
+        let types = fallback_game_types();
+        let ids: Vec<&str> = types.iter().map(|g| g.id.as_str()).collect();
+        assert_eq!(
+            vec!["scopone_scientifico", "briscola", "briscola_a_quattro"],
+            ids
+        );
+        // The lobby's first game (the creation default) must exist.
+        assert_eq!("scopone_scientifico", types[0].id);
+    }
+
     #[test]
     fn game_view_parses_briscola_payload() {
         let g: GameView = serde_json::from_value(json!({