docs: document app-only compose + fix dev.sh for relay profile

dev.sh still has to opt into the relay profile now that it's no
longer in the default set; otherwise `./dev.sh` would print "relay:
http://localhost:8081" but skip starting the container. README gets
a short compose section in Quick Start explaining the app-only
default and how to add --profile relay for a local relay.

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

dev.sh

@@ -1,9 +1,11 @@
 #!/bin/bash
-# Start hyprcrd + relay for local development
+# Start hyprcrd + relay for local development. The relay service is
+# behind a compose profile since it's opt-in — this script explicitly
+# opts in so you get both containers.
 
 cd "$(dirname "$0")"
 
-docker compose up -d --build
+docker compose --profile relay up -d --build
 
 echo ""
 echo "  hyprcrd:  http://localhost:8088"
@@ -11,3 +13,5 @@ echo "  relay:    http://localhost:8081"
 echo ""
 echo "  relay from hyprcrd container: http://relay:80"
 echo ""
+echo "  (for app-only, drop the relay profile:  docker compose up -d)"
+echo ""