v1.1.0 — Gzip canvas compression + throttled sends

- Canvas snapshots gzip-compressed before sending: 1MB raw -> ~1-5KB
  for a mostly-white canvas, ~100-500KB for a busy one
- Semaphore throttles concurrent canvas sends (default 20) to prevent
  memory spikes during connection bursts
- Frontend decompresses gzip via browser DecompressionStream API
- New config: MAX_CONCURRENT_CANVAS_SENDS (default 20)
- Added flate2 dependency for gzip encoding

crates/config/src/lib.rs

@@ -24,6 +24,7 @@ pub struct ServerConfig {
     pub address: String,
     pub port: u16,
     pub enable_cors: bool,
+    pub max_concurrent_canvas_sends: usize,
 }
 
 #[derive(Debug, Clone)]