feat: implement dark mode token variants

[davidawele3]

• useDarkMode: set data-theme='dark' on instead of class toggle
• App.jsx: remove duplicate inline nav, delegate to NavBar component
• index.css: fix light-mode accent (#0369a1) and success (#15803d) to meet WCAG AA 4.5:1 contrast on white
• Replace all hardcoded colors with CSS variables in: AnalyticsDashboard, PatientDashboard, IssuerDashboard, AdminDashboard, Landing, VerifyPage, FreighterBanner
• Add darkMode.test.js: data-theme attribute + WCAG AA contrast checks (27 tests passing)

Closes #277

Description

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Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

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Testing

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• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests passed locally with my changes
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• If this PR adds a new directory or key file, I have updated docs/folder-structure.md.

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