feat: add centralized toast notification system with ToastContext, Toast component, and integrated error displays

[waterWang]

Summary

Implements Issue #157 — Creates a centralized toast/notification system to replace the inconsistent inline error/success message patterns scattered across components.

Changes

New files

• frontend/src/context/ToastContext.tsx — ToastProvider with queue management, useToast hook, and ToastContainer component
• frontend/src/components/common/Toast.css — Toast styles with slide-in animation, type variants (success/error/warning/info), and responsive mobile layout

Modified files

• frontend/src/App.tsx — Wrap app with ToastProvider and import Toast.css
• frontend/src/components/agents/TaskSubmissionForm.tsx — Replace inline fixed-position error div with centralized useToast('error')
• frontend/src/pages/dashboard.tsx — Add error toast when network stats fetch fails
• frontend/src/components/dashboard/RecentTasksTable.tsx — Replace silent console.error with visible error toast

Acceptance Criteria

• ✅ Toast notifications appear in the top-right corner with slide-in animation
• ✅ Four toast types: success (green), error (red), warning (amber), info (blue)
• ✅ Auto-dismiss after 5 seconds with manual dismiss button
• ✅ TaskSubmissionForm shows error toast instead of inline error div
• ✅ Dashboard shows error toast on network stats fetch failure
• ✅ RecentTasksTable shows error toast on fetch failure instead of silent console.error
• ✅ Responsive: full-width on mobile (< 480px)

Testing

• TypeScript compiles without new errors (pre-existing @stellar/stellar-sdk errors are unrelated)

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[devJaja]

Solid implementation @waterWang
make the CI frontend to passed

[devJaja]

@waterWang

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