Update favicon with custom bill-splitting icon

[petercort]

Update Favicon with Custom Bill-Splitting Icon

This PR implements a custom favicon that better represents the Split the Trip application's functionality.

Changes Made

🎨 New Favicon Design

• Custom SVG: Created a clean, modern favicon showing a receipt/bill with a dollar sign and dots representing multiple people splitting costs
• Theme Consistency: Used the app's color scheme (indigo/purple) to match the overall design
• Recognizable at Small Sizes: Designed to be clear and identifiable even in browser tabs

📁 Multiple Format Support

• favicon.svg - Vector format for modern browsers
• favicon-16x16.png - Standard small size for compatibility
• favicon-32x32.png - Larger size for high-DPI displays
• favicon.ico - Updated traditional format in app directory

🔧 Technical Improvements

• Enhanced Metadata: Updated layout.tsx with proper favicon metadata including all formats
• Better Browser Support: Multiple formats ensure compatibility across all browsers
• Optimized Generation: Used Sharp library to generate high-quality PNG versions from SVG

Visual Features

The new favicon includes:

• A receipt/bill document icon in white on an indigo background
• Dollar sign symbol to represent money/expenses
• Three dots at the bottom representing multiple people sharing the bill
• Clean, minimalist design that's professional and recognizable

Testing

• ✅ Favicon displays correctly in browser tabs
• ✅ All formats generate without errors
• ✅ Proper metadata implementation in Next.js
• ✅ Matches app's overall design theme

Resolves

Closes #13

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The favicon now clearly communicates the app's purpose of splitting expenses among group members, making it more professional and recognizable!