Added empty state component for zero activiities

[VaibhaviSuvarna]

feat: Implement Empty State Component for User Engagement

Description:
This PR introduces a new EmptyState component to enhance the user experience for new users or those with no recorded activities. Previously, the dashboard would appear blank or with default data, which could be unengaging. This new component provides a friendly and encouraging message, guiding users to start tracking their carbon footprint.

Problem Solved:
Addresses the lack of a clear onboarding or guidance mechanism for users without existing data, improving initial user engagement and retention.

Files Modified:

• src/components/ui/EmptyState.tsx: New component file.
• src/components/dashboard/Dashboard.tsx: Modified to import and utilize the EmptyState component.

Acceptance Criteria Met:

• Component displays when no activities exist (integrated into Dashboard.tsx).
• Friendly, encouraging message (e.g., "Start Your Green Journey!").
• Clear call-to-action button (e.g., "➕ Log Your First Activity").
• Centered layout with proper spacing.
• Responsive design (achieved through Tailwind CSS).
• Matches app's eco-friendly theme.
• Can be reused for different empty states.

This PR CLOSES issue #53