Installer: respect a declined Studio auto-start and keep Ctrl+C shutdown logs ordered

The `curl | sh` Studio auto-start prompt had two issues on Linux/macOS/WSL
(install.sh). install.ps1 already gates on input redirection, so Windows is
unaffected.

1. Typing n, or any closed/EOF /dev/tty, still launched Studio. The read
   fallbacks defaulted to "y" (read failure, and the no-tty branch), so any
   answer other than a cleanly delivered y/n line auto-started a blocking
   foreground server. Default those to "n"; a real Enter still counts as yes
   via ${_reply:-y}.

2. On Ctrl+C the shell prompt printed in the middle of Studio's shutdown logs.
   The non-interactive installer shell took the default SIGINT action and died
   before the child finished its graceful shutdown, so the prompt raced ahead
   of "All subprocesses cleaned up". trap '' INT in the installer shell so it
   waits for Studio's own graceful shutdown.

install.sh

@@ -3126,10 +3126,13 @@ echo ""
 if [ -t 1 ]; then
     echo ""
     printf "  Start Unsloth Studio now? [Y/n] "
+    # Default to NOT starting when no answer can be read (closed/EOF tty) so a
+    # non-interactive caller is never trapped in a foreground server. A real
+    # Enter still counts as yes via ${_reply:-y} below.
     if [ -r /dev/tty ]; then
-        read -r _reply </dev/tty || _reply="y"
+        read -r _reply </dev/tty || _reply="n"
     else
-        _reply="y"
+        _reply="n"
     fi
     case "${_reply:-y}" in
         [Yy]*|"")
@@ -3137,6 +3140,9 @@ if [ -t 1 ]; then
             # Detach stdin from the `curl | sh` pipe: as a foreground server the
             # studio would otherwise drain the rest of this piped script, leaving
             # the shell to die parsing the now-truncated tail (`unexpected fi`).
+            # Ignore Ctrl+C so this shell waits for studio's own graceful
+            # shutdown instead of dying first and racing the prompt over its logs.
+            trap '' INT
             "$VENV_DIR/bin/unsloth" studio -p 8888 </dev/null
             _LAUNCH_EXIT=$?
             if [ "$_LAUNCH_EXIT" -ne 0 ] && [ "$_MIGRATED" = true ]; then