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.lock

@@ -7522,7 +7522,7 @@ name = "solitaire_core"
 version = "0.1.0"
 dependencies = [
  "chrono",
- "rand 0.8.6",
+ "rand 0.9.4",
  "serde",
  "thiserror 2.0.18",
 ]