Modernize the Frontend UI

[hk2166]

Please prefix your pull request with one of the following:
[Feature Request] Modernize the Frontend UI #1062

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This PR implements the frontend modernization proposed in the issue.
The outdated Zurb Foundation framework, Font Awesome 4.x, and jQuery
have been fully replaced with a modern, maintainable stack.

Added

• static/css/modern.css — a new design system built with CSS custom
  properties (variables), covering layout, typography, components,
  dark mode, and responsive behavior
• Google Fonts (Inter) for modern typography
• Font Awesome 6 (via CDN) replacing FA 4.x

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 Security Hotspot
E Security Rating on New Code (required ≥ A)
C Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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