feat: add toast notification system (Closes #825) [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Bounty: Toast Notification System (Closes #825)

Implements a global toast notification system for bounty status changes, errors, and success messages.

Changes

frontend/src/contexts/ToastContext.tsx (new)

• ToastProvider — global context provider rendering a top-right toast stack.
• useToast() hook exposing success, error, warning, info, and dismiss.
• Toast component with 4 variants (success/error/warning/info), each with a distinct lucide icon and border color.
• Auto-dismiss after 5 seconds with manual close button.
• Slide-in spring animation from top-right via framer-motion.
• Multiple toasts stack vertically.
• Accessible via role=\alert\ and an aria-live=\polite\ container.

frontend/src/main.tsx

• Wrapped the app in ToastProvider.

frontend/src/components/bounty/BountyCreateWizard.tsx

• Integrated toasts into the bounty creation flow: success toast on draft creation and publish, error toast on failure.

frontend/src/__tests__/toast.test.tsx (new)

• Verifies all 4 variants render.
• Verifies auto-dismiss after 5s.
• Verifies manual dismiss via close button.
• Verifies multiple toasts stack.
• Verifies role=\alert\ accessibility.

Verification

• vitest run src/__tests__/toast.test.tsx — 8 tests pass.
• No new TypeScript errors introduced (pre-existing lib/animations module errors are unrelated).