feat: relay ping handling, identity persistence, linux build script (backport)

Backported from feat/android-voip-client:
- Relay: SNI "ping" connections handled gracefully (no timeout errors)
- Relay: identity persisted in ~/.wzp/relay-identity (stable fingerprint)
- Linux fire-and-forget build script (Hetzner VM)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/Cargo.toml

@@ -28,6 +28,7 @@ prometheus = "0.13"
 axum = { version = "0.7", default-features = false, features = ["tokio", "http1", "ws"] }
 tower-http = { version = "0.6", features = ["fs"] }
 futures-util = "0.3"
+dirs = "6"
 
 [[bin]]
 name = "wzp-relay"