Replace native `confirm()` dialogs with the app's Dialog component

[Emp1500]

What happened?

Three places still use the browser's native confirm():

• components/dashboard/FlowchartCard.tsx — delete confirmation
• components/editor/EditorToolbar.tsx — "Save before leaving?" and delete

Native dialogs clash with the app's dark theme and can't be styled. The
project already ships components/ui/dialog.tsx (shadcn) — use it for a
proper confirmation dialog with a destructive-styled action button.

Code snippet (if parser-related)

Environment

No response

Where did this happen?

Live site (code2flow-one.vercel.app)