test(e2e-ui): print the proxy log when the proxy dies on its own

In E2E_KEEP_ALIVE=1 mode the harness blocks until the proxy pid goes away, then
printed a bare "Proxy exited." and fell straight into cleanup, which rm -f's the
log. The proxy has now exited by itself twice, minutes after a run had finished,
leaving nothing to look at. Both startup failure paths already tail -n 100 the log
before giving up, so this was the one death that stayed silent

Dump the same 100 lines before exiting. A normal Ctrl-C teardown still deletes the
log and prints nothing, which is why INT and TERM now exit instead of running
cleanup and falling back into the wait loop: under the single trap a SIGTERM
deleted the log, resumed the loop, and would then report "tail: no such file",
besides running cleanup twice

tests/e2e/ui/run_e2e.sh

@@ -89,7 +89,11 @@ cleanup() {
   fi
   echo "Done."
 }
-trap cleanup EXIT INT TERM
+on_signal() {
+  exit 130
+}
+trap cleanup EXIT
+trap on_signal INT TERM
 
 # --- Pre-flight checks ---
 for cmd in python3 npx uv; do
@@ -271,7 +275,8 @@ EOF
   while kill -0 "$PROXY_PID" 2>/dev/null; do
     sleep 5
   done
-  echo "Proxy exited."
+  echo "Error: proxy process exited unexpectedly. Proxy output:"
+  tail -n 100 "$PROXY_LOG"
   exit 1
 fi