fix: use printf instead of echo in StackBlitz start command

Replace echo with printf and proper escape sequences to avoid
shell parsing issues in StackBlitz's WebContainer environment.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: katspaugh

.stackblitzrc

@@ -1,6 +1,6 @@
 {
   "installDependencies": true,
-  "startCommand": "npm run build && npm link && echo '\nSafe CLI is ready! Try running: safe --help\n'",
+  "startCommand": "npm run build && npm link && printf '\\n✓ Safe CLI is ready! Try running: safe --help\\n\\n'",
   "env": {
     "NODE_ENV": "development"
   },