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/src/js/terminal.js

@@ -393,6 +393,10 @@ class TerminalManager {
                 label: 'Find…',
                 action: () => this.app._openTerminalSearch(),
             },
+            {
+                label: 'Send Snippet…',
+                action: () => this.app.snippets.open(),
+            },
             { divider: true },
             {
                 label: 'Clear Terminal',