feat: add high-contrast theme mode (DS-066)

[precious-akpan]

Summary

Adds an optional high-contrast mode that works independently of light/dark theme selection to meet diverse user contrast needs.

Changes

• Extended ThemeProvider with highContrast state and persistence
• Added .high-contrast CSS class with WCAG AAA contrast overrides
• Boosted text, border, and focus indicator contrast
• Enhanced interactive state visibility
• Maintains distinguishable charts and trading states without oversaturation
• Applies before first paint to avoid contrast flash

Implementation

• highContrast boolean stored in localStorage (so4-high-contrast)
• CSS variables overridden for both light and dark modes
• Focus indicators remain clearly visible on all surfaces
• All semantic tokens maintain meaning in high-contrast mode

Tokens Modified

• --text-primary, --text-secondary, --text-tertiary
• --border, --input, --ring
• --primary, --destructive
• --success, --warning, --info

Future Work

• Add high-contrast toggle to appearance settings panel
• Add automated contrast ratio tests
• Document which tokens change in high-contrast mode

Closes #466

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