Add snippet/command library with CRUD and send-to-session

Full command snippet library for saving and sending reusable commands:

Backend:
- SQLite migration 003 creates snippets table
- Snippet model with name, command, description, folder, tags
- Store CRUD methods and API endpoints

Frontend:
- SnippetLibrary class with searchable list + detail/editor split view
- Create, edit, delete snippets in modal dialog
- 'Send to Session' button writes command to active terminal
- 'Send Snippet' option in terminal right-click context menu
- Ctrl+Shift+S keyboard shortcut to open library

frontend/index.html

@@ -212,6 +212,7 @@
     <script src="/src/js/recordings.js"></script>
     <script src="/src/js/sftp.js"></script>
     <script src="/src/js/splitpane.js"></script>
+    <script src="/src/js/snippets.js"></script>
     <script src="/src/js/app.js"></script>
 </body>
 </html>