fix: web audio capture buffer size + relay warning

- Use power-of-2 buffer (1024) for ScriptProcessorNode
- Accumulate samples and send exact 960-sample frames
- Remove unused watch import from relay

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

crates/wzp-relay/src/main.rs

@@ -9,7 +9,7 @@ use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::Arc;
 use std::time::Duration;
 
-use tokio::sync::{Mutex, watch};
+use tokio::sync::Mutex;
 use tracing::{error, info, warn};
 
 use wzp_proto::MediaTransport;