feat: add light/dark theme toggle with system-preference default

[favasconcelos]

Summary

• Adds a light theme to the site alongside the existing dark theme.
• Adds a fixed top-right light/dark toggle.
• Defaults to the user’s system preference (prefers-color-scheme) and persists an explicit choice in localStorage.
• Updates CSS tokens to use Tailwind CSS v4 @theme inline with a custom data-theme dark variant.
• Adjusts the hero image/overlay for light mode.

Changes

• src/hooks/use-theme.ts — theme state, system-preference fallback, localStorage persistence.
• src/components/theme-toggle.tsx — accessible toggle button.
• src/index.css — light/dark token sets.
• index.html — inline theme initializer to prevent FOUC.
• src/app.tsx — integrates the toggle.
• src/components/hero.tsx — light-mode image opacity and overlay.

Validation

• npm run check passes.
• npm run build passes.

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