ui: Replace browser alerts with custom modern toast notifications.

[Prakshil]

Related Issue

Closes #257

Summary

This PR modernizes the notification system by completely replacing the project's native browser alert() and confirm() dialogs with a custom, zero-dependency Vanilla JavaScript toast notification and modal system. This significantly improves the UI/UX consistency, maintains the project's glassmorphism aesthetic, and avoids the jarring flow interruptions caused by default browser dialogs.

Changes Made

• Created a Custom Notification System (toast.js): Implemented a lightweight ToastManager class to handle both slide-in toasts and confirmation modals without relying on heavy external React libraries.
• Removed Blocking Alerts: Refactored over a dozen synchronous alert() calls across store.js and app.js, replacing them with non-blocking Toast.show() calls for success, error, warning, and info messages.
• Modernized Confirmations: Replaced the native confirm() dialog for task deletion with an asynchronous, beautifully styled await Toast.confirm() modal.
• Enhanced Styling (index.css): Added styles and @keyframes animations for top-right sliding toasts and a fading modal backdrop. The components use existing CSS variables to automatically adapt to the application's Light/Dark mode toggles.
• Updated Documentation: Added the new "Smart Notifications" feature to the README.md and updated the project structure tree.

Testing

• Validation Testing: Attempted to add an empty task and verified that the warning toast slides in from the top-right and auto-dismisses.
• Interaction Testing: Finished a Focus session to ensure the success toast correctly fires.
• Promise Resolution Testing: Clicked "delete" on a task to ensure the new custom confirm modal appears, intercepts the action, and accurately resolves the promise based on clicking "Confirm" or "Cancel".
• Theme Testing: Toggled between Light Mode and Dark Mode to verify that the toast and modal backgrounds/text colors adapt seamlessly to the CSS variables.

Screenshots

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)

[Prakshil]

I have successfully implemented the Notification toasts , please review it and merge it.
thank you.