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

internal/models/models.go

@@ -256,3 +256,15 @@ type AuditEntry struct {
 	SessionID string    `json:"sessionId,omitempty"`
 	Details   string    `json:"details"`
 }
+
+// Snippet stores a reusable command for the snippet library.
+type Snippet struct {
+	ID          string    `json:"id"`
+	Name        string    `json:"name"`
+	Command     string    `json:"command"`
+	Description string    `json:"description"`
+	Folder      string    `json:"folder"`
+	Tags        []string  `json:"tags"`
+	CreatedAt   time.Time `json:"createdAt"`
+	UpdatedAt   time.Time `json:"updatedAt"`
+}