[shogun444]

Description

What changed?

• Added a React Context-based toast system exposing ToastProvider and useToast() in mobileapp/src/components/Toast.tsx.
• Supported toast variants: success, error, warning, and info.
• Added backward compatibility so existing global.toast callers continue working, with proper cleanup on unmount.
• Wrapped root layout with <ToastProvider> in mobileapp/app/_layout.tsx and removed the redundant <ToastManager />.

Why was it changed?
To modernize toast notifications by moving away from mutating global state (global.toast) toward a type-safe, idiomatic React Context and hook pattern.

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Type of Change

• Refactor / Code modernization
• New feature (non-breaking)
• Bug fix
• Breaking change

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Verification

• Backward Compatibility: All 7 existing global.toast call sites continue to work without modification.
• Lifecycle Cleanup: Verified global.toast references are cleanly removed on unmount.
• Layout Integration: Root layout cleanly wraps children with zero dangling ToastManager references.
• Zero Dependencies: Implemented purely with React built-ins.

Closes #712

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[ONEONUORA]

@shogun444
PLs make your implementation to pass CI checks

[ONEONUORA]

Great job @shogun444