chore(workspace): upgrade rand 0.9, edition 2024, expand server tests

- rand "0.8" → "0.9": StdRng/SliceRandom API unchanged; 142 core tests pass
- edition "2021" → "2024" workspace-wide: no gen keyword conflicts found;
  204 tests (core + sync) pass clean with zero warnings
- ARCHITECTURE.md: Edition 2021 → Edition 2024 in header
- solitaire_server tests: add 5 new integration tests covering
  refresh-with-garbage-token, expired-refresh-token, push-without-token,
  delete-account-without-token, and leaderboard-authenticated-but-empty

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

Cargo.toml

@@ -11,7 +11,7 @@ members = [
 resolver = "2"
 
 [workspace.package]
-edition = "2021"
+edition = "2024"
 version = "0.1.0"
 license = "MIT"
 
@@ -21,7 +21,7 @@ serde_json  = "1"
 uuid        = { version = "1", features = ["v4", "serde"] }
 chrono      = { version = "0.4", features = ["serde"] }
 thiserror   = "2"
-rand        = "0.8"
+rand        = "0.9"
 async-trait = "0.1"
 tokio       = { version = "1", features = ["full"] }
 dirs        = "6"