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/api.rs

@@ -152,8 +152,10 @@ pub async fn my_ratings() -> Result<RatingsPage, String> {
     resp.json().await.map_err(|e| e.to_string())
 }
 
-pub async fn leaderboard() -> Result<LeaderboardPage, String> {
-    let resp = Request::get("/api/leaderboard")
+/// Fetch the leaderboard for one game type: Elo, wins, matches and points
+/// are all scoped to that game.
+pub async fn leaderboard(game_type: &str) -> Result<LeaderboardPage, String> {
+    let resp = Request::get(&format!("/api/leaderboard?game_type={game_type}"))
         .send()
         .await
         .map_err(|e| e.to_string())?;