[#204 and #205 ] Fixing bugs for line and bar charts + minor changes

🎯 Purpose

1. Fixing the charts bugs
2. Removing unused account modal
3. Updated Federico's link in the Contributors page

📱 Changes Made

This PR contains the following changes:

• Fixed line chart empty spots issue by adding the option to backfill the data provided to the widget
• Fixed the average shown in the bar chart, now it looks for the first month of tracking as a starting point and divides by the month that passed since instead of always dividing by 12
• Deleted account_modal.dart since is not used in the app
• Updated Federico's link to his portfolio instead of his GitHub

🧪 Testing Instructions

• For line chart see the steps described in Bug: Line chart has empty spots in days in which there are no expenses #204
• For line chart see the steps described in Bug: Average in bar chart for income/expenses is wrong #205
• For Federico's link: check that the links redirects to his website

🔍 Review Checklist

• Code follows project style guidelines
• Documentation updated (if needed)
• Widgets are properly disposed