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/app.js

@@ -4,7 +4,7 @@
  * Initialises the application shell, loads profiles, and sets up event listeners.
  */
 
-const { State, Sidebar, TabManager, TerminalManager, ProfileEditor, RawViewer, Notifications, Settings, CommandPalette, SecretPrompt, SplitPaneManager, RecordingManager, ExportImportUI, SFTPBrowser, stateLabel } = window.NexusTerminal;
+const { State, Sidebar, TabManager, TerminalManager, ProfileEditor, RawViewer, Notifications, Settings, CommandPalette, SecretPrompt, SplitPaneManager, RecordingManager, ExportImportUI, SFTPBrowser, SnippetLibrary, stateLabel } = window.NexusTerminal;
 
 class App {
     constructor() {
@@ -22,6 +22,7 @@ class App {
         this.recordings = new RecordingManager(this);
         this.exportImport = new ExportImportUI(this);
         this.sftp = new SFTPBrowser(this);
+        this.snippets = new SnippetLibrary(this);
     }
 
     /**
@@ -392,6 +393,13 @@ class App {
                 return;
             }
 
+            // Ctrl/Cmd+Shift+S — open snippet library
+            if (mod && e.shiftKey && e.key === 'S') {
+                e.preventDefault();
+                this.snippets.open();
+                return;
+            }
+
         });
     }