dev: dev.sh detects stale servers and offers to kill them

Before starting, each port (8080, 5173) is checked via ss; an existing
listener is reported with PID and command, and an interactive run asks
permission to kill it (default yes). Non-interactive runs abort
instead: never kill processes silently. Born from a real incident: an
orphaned smoke-test server held 8080 and dev.sh only said
'address already in use'.

dev.sh

@@ -3,10 +3,44 @@
 #   - Go API on localhost:8080
 #   - Vite dev server on localhost:5173 (proxies /api to the Go server)
 # One Ctrl-C stops everything; if either server dies, the other is stopped.
+# If a port is already taken (usually a stale server), offers to kill it.
 set -euo pipefail
 
 cd "$(dirname "$0")"
 
+# free_port checks whether something is listening on the port and, when run
+# interactively, offers to kill it. Otherwise it refuses to continue: never
+# kill processes silently.
+free_port() {
+  local port="$1" name="$2"
+  local listener_pid
+  listener_pid=$(ss -tlnp "sport = :$port" 2>/dev/null | grep -oP 'pid=\K[0-9]+' | head -1 || true)
+  [ -z "$listener_pid" ] && return 0
+
+  local listener_cmd
+  listener_cmd=$(ps -p "$listener_pid" -o comm= 2>/dev/null || echo unknown)
+  echo "dev.sh: port $port ($name) is already in use by PID $listener_pid ($listener_cmd)."
+
+  local answer="n"
+  if [ -t 0 ]; then
+    read -r -p "Kill it and continue? [Y/n] " answer
+    answer=${answer:-y}
+  fi
+  case "$answer" in
+    [Yy]*)
+      kill "$listener_pid"
+      sleep 0.5
+      ;;
+    *)
+      echo "dev.sh: aborting; free port $port first."
+      exit 1
+      ;;
+  esac
+}
+
+free_port 8080 "Go API"
+free_port 5173 "Vite"
+
 if [ ! -d web/node_modules ]; then
   echo "dev.sh: first run, installing frontend dependencies..."
   (cd web && npm install)