feat: implement risk trends dashboard with charts and comparisons

[awwalmust]

Summary

Adds a Risk Trends Dashboard page that allows organizations to visually
track and compare risk levels over time across departments.

Changes Made

• Created apps/web/app/risk-trends/page.tsx — new Next.js page for
  the risk trends dashboard
• Implemented Historical Risk Chart using dynamic CSS bars with severity
  colour coding (green = low, yellow = medium, red = high)
• Added Risk Comparisons section showing department scores with
  percentage trend indicators (up/down/stable)
• Added Time Filter dropdown with 7d, 30d, 90d, and 1y options
• Responsive grid layout across all screen sizes

Why

Provides a visual and measurable way to track organizational risk over
time, making it easier to monitor security improvements and compare
risk levels across different departments.

How It Was Tested

• Page renders correctly without syntax errors
• Grid layouts confirmed responsive across screen sizes
• Time filter dropdown state changes work correctly

Screenshots

Closes #154

[mijinummi]

Hello @awwalmust , thanks for contributing, please kindly fix the remaining linting failures. Well done for the effort you've put on so far