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-client/Cargo.toml

@@ -50,8 +50,9 @@ rustls        = { workspace = true }
 tracing       = { workspace = true }
 tracing-subscriber = { workspace = true }
 
-# CLI
+# CLI + config
 clap          = { workspace = true }
+toml          = { workspace = true }
 
 # Local message/conversation storage
 rusqlite      = { workspace = true }