chore(deps): migrate to Bevy 0.16, axum 0.8, and other package updates

- Bump bevy 0.15 → 0.16; fixes all breaking API changes:
  ChildBuilder → ChildSpawnerCommands, Parent → ChildOf,
  despawn_descendants → despawn_related::<Children>(),
  despawn_recursive → despawn (now recursive by default),
  EventWriter::send → write, Query::{get_single,get_single_mut}
  → {single,single_mut}, ChildOf::get → parent()
- Bump axum 0.7 → 0.8; remove axum::async_trait from FromRequestParts
- Bump tower_governor 0.4 → 0.8; fix GovernorLayer::new() API
- Bump jsonwebtoken 9 → 10 with rust_crypto feature only
- Bump thiserror 1 → 2, dirs 5 → 6, bcrypt 0.15 → 0.19,
  reqwest 0.12 → 0.13 (rustls feature rename)
- Regenerate .sqlx offline cache for sqlx compile-time query checks

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/challenge_plugin.rs

@@ -59,8 +59,8 @@ fn advance_on_challenge_win(
         }
         // Human-readable level is 1-based (index 0 → "Challenge 1").
         let level_number = prev.saturating_add(1);
-        toast.send(InfoToastEvent(format!("Challenge {level_number} complete!")));
-        advanced.send(ChallengeAdvancedEvent {
+        toast.write(InfoToastEvent(format!("Challenge {level_number} complete!")));
+        advanced.write(ChallengeAdvancedEvent {
             previous_index: prev,
             new_index: progress.0.challenge_index,
         });
@@ -77,7 +77,7 @@ fn handle_start_challenge_request(
         return;
     }
     if progress.0.level < CHALLENGE_UNLOCK_LEVEL {
-        info_toast.send(InfoToastEvent(format!(
+        info_toast.write(InfoToastEvent(format!(
             "Challenge mode unlocks at level {CHALLENGE_UNLOCK_LEVEL}"
         )));
         return;
@@ -86,7 +86,7 @@ fn handle_start_challenge_request(
         warn!("challenge seed list is empty");
         return;
     };
-    new_game.send(NewGameRequestEvent {
+    new_game.write(NewGameRequestEvent {
         seed: Some(seed),
         mode: Some(GameMode::Challenge),
     });