[app-bug] No global toast/notification system — errors render as inline strings only

[Gbangbolaoluwagbemiga]

Problem

Every page invents its own way to report errors (inline string under the upload box, an amber banner on /terminal, an alert() in /playground's reset action). There is no app-wide toast/snackbar, so concurrent errors collide and silent successes have no positive feedback.

Acceptance Criteria

• Add a small <ToastProvider> (no library required — ~50 LOC) in frontend/app/providers/
• Expose a useToast() hook with toast.error(), toast.success(), toast.info()
• Position top-right, stack up to 3, auto-dismiss after 4s, dismissible by click
• Replace existing inline error renders on /scan, /dashboard, /playground, /terminal with toast calls
• Storybook story for the provider with the three variants

Pointers

• frontend/app/providers/
• frontend/app/scan/page.tsx (inline error)
• frontend/app/terminal/page.tsx (amber banner)
• frontend/app/playground/page.tsx (confirm()/alert() usage)

Difficulty

Easy. No external dependency needed.