feat: add light theme parity and persistence

[lamborghini21]

feat: add light theme parity and persistence

• Implement CSS variable design tokens
• Add theme toggle in header
• Persist theme preference in localStorage
• Default to prefers-color-scheme
• Add smooth theme transitions
• Prevent first-paint flash
• Support light/dark RainbowKit themes
• Ensure WCAG AA contrast compliance

Closes #71

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[truthixify]

Clean merge. Light theme with CSS variables, header toggle, localStorage persistence, prefers-color-scheme default, and FOUC prevention all land cleanly. The pnpm-workspace.yaml allowBuilds block also fixes a pnpm install warning. Thanks @lamborghini21.