Add workspace deletion with confirmation dialog

[Vrutin]

This PR adds the ability to delete workspaces from CodexMonitor.

Changes

• Added removeWorkspace service function in tauri.ts that calls the backend remove_workspace command
• Added removeWorkspace hook function in useWorkspaces.ts with confirmation dialog using Tauri's ask API
• Added workspace context menu in Sidebar.tsx with delete option (right-click on workspace)
• Wired up delete handler in App.tsx

Features

• Right-click any workspace in the sidebar to access the context menu
• Select "Delete" to remove a workspace
• A confirmation dialog appears with "Delete" and "Cancel" buttons
• The dialog shows the workspace name and warns about removal
• Only proceeds with deletion if user confirms

Technical Details

• Uses Tauri's native ask dialog for platform-native confirmation UI
• Properly cleans up workspace state and active workspace selection
• Includes debug logging for deletion events
• Follows the same pattern as thread deletion for consistency