feat: add client auto-reconnect, heartbeat, and connection status UI

RPC layer (quicprochat-rpc):
- RpcClient now uses tokio::sync::Mutex<Connection> for safe reconnection
- Auto-reconnect with exponential backoff + jitter on retriable errors
- QUIC-level keepalive via quinn TransportConfig
- subscribe_push() returns Option<PushFrame> with None sentinel on break
- RpcError::is_retriable() classifies transient vs permanent errors
- ConnectionState enum (Connected/Reconnecting/Disconnected) with Display
- Configurable max_retries, base_delay, max_backoff, keepalive_secs

SDK layer (quicprochat-sdk):
- QpqClient wraps RpcClient in Arc for safe heartbeat task sharing
- start_heartbeat() spawns background task checking connection every 30s
- connection_state() exposes RPC-layer state to UI
- Reconnecting event added to ClientEvent enum
- disconnect() aborts heartbeat before closing connection

Client UI (quicprochat-client):
- TUI status bar shows Connected/Reconnecting.../Offline with color
- TUI handles Reconnecting event with attempt count display
- REPL event listener prints connection state changes
- REPL /status shows connection state instead of bool
- Both TUI and REPL call start_heartbeat() on startup

Cargo.lock

@@ -4491,6 +4491,7 @@ dependencies = [
  "prost",
  "quicprochat-proto",
  "quinn",
+ "rand 0.8.5",
  "rcgen",
  "rustls",
  "sha2 0.10.9",