feat(toast): add toast notification system (#825)

[wengkit218-pixel]

Summary

Implements a complete toast notification system for the SolFoundry frontend.

Changes

New: ToastContext.tsx

• ToastProvider — Context provider wrapping the app, renders a fixed toast container (top-right)
• useToast() — Hook returning toast(message, variant) for triggering notifications anywhere
• 4 variants: success (emerald), error (red), warning (amber/tier-t2), info (blue)
• Auto-dismiss after 5 seconds with manual close button (X icon)
• Slide-in animation from right via framer-motion (AnimatePresence + layout)
• Stacking — Multiple toasts stack vertically with gap
• Accessible — Each toast has role="alert" for screen readers
• Uses existing project design tokens (bg-emerald, border-border, text-text-muted, etc.)

Modified: main.tsx

• Added ToastProvider inside AuthProvider (so auth-dependent components can use it)

New: __tests__/ToastContext.test.tsx

• Tests for all 4 variants rendering
• Auto-dismiss timing (5s)
• Manual dismiss via close button
• Multiple toast stacking
• Error when used outside provider

Integration Example

import { useToast } from '../contexts/ToastContext';

function SubmitButton() {
  const { toast } = useToast();

  const handleSubmit = async () => {
    try {
      await api.submit(data);
      toast('Submission successful!', 'success');
    } catch {
      toast('Submission failed. Try again.', 'error');
    }
  };
}

Acceptance Criteria Met

• Toast notifications appear for key actions
• Auto-dismiss with manual close option
• Multiple toasts stack properly
• Slide-in animation from top-right
• Accessible (role="alert")

Closes #825

[wengkit218-pixel]

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