feat: upgrade OpenMLS 0.5 → 0.8 for security patches and GREASE support

Migrates all MLS code in quicprochat-core from OpenMLS 0.5 to 0.8:
- StorageProvider replaces OpenMlsKeyStore (keystore.rs full rewrite)
- HybridCryptoProvider updated for new OpenMlsProvider trait
- Group operations updated for new API signatures
- MLS state persistence via MemoryStorage serialization
- tls_codec 0.3 → 0.4, openmls_traits/rust_crypto 0.2 → 0.5

crates/quicprochat-server/Cargo.toml

@@ -65,7 +65,7 @@ serde_json    = { workspace = true }
 
 # CLI
 clap          = { workspace = true }
-toml          = { version = "0.8" }
+toml          = { workspace = true }
 
 # WebSocket JSON-RPC bridge for browser clients
 tokio-tungstenite = "0.26"