Update frontend styling to match GenomicX design system

[happykhan]

Summary

• Replaces all --color-* CSS custom properties with --gx-* tokens from the GenomicX front-end template
• Switches to dark-first theme design with system preference auto-detection
• Aligns color palette with the standard Tailwind Slate/Teal/Indigo scale used across GenomicX apps
• Updates responsive breakpoint from 700px to 768px to match template
• Updates footer to match the shared GenomicX footer pattern

Modified Files

• src/index.css — Complete replacement with GenomicX design tokens + app-specific allele badge tokens
• src/App.css — All --color-* references replaced with --gx-* equivalents
• src/App.tsx — Dark theme default, updated localStorage key, shared footer

Test Plan

• Verify dark theme loads by default
• Toggle between dark/light themes
• Check all allele badges (exact/novel/no-hit) render correctly in both themes
• Verify responsive layout at mobile widths (< 768px)
• Run MLST analysis end-to-end to confirm no functional regressions
• Verify footer links work correctly

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