Respect system theme preference

[GHX5T-SOL]

Refs #222.

Summary

• switch Tailwind dark variants to class-based mode so they follow the root theme class
• choose the initial root .dark class before CSS loads from localStorage.theme or prefers-color-scheme: dark
• remove the hard-coded dark body class and add color-scheme for light/dark tokens

Validation

• node --check assets/tailwind.config.js
• mix format --check-formatted lib/algora_web/components/layouts/root.html.heex assets/css/app.css
• mix compile (passes; existing unrelated warnings in current main)
• mix tailwind algora
• git diff HEAD~1..HEAD --check
• git diff HEAD~1..HEAD | gitleaks detect --pipe --no-banner --redact (no leaks found)

Notes

• This is the system-theme/localStorage foundation for Enhancement: Add support for system dark mode via prefers-color-scheme or manual theme toggle #222; it does not add the visible manual toggle UI.
• mix assets.build did not complete locally because pnpm/Corepack blocked ignored dependency build scripts before Tailwind ran, so I validated Tailwind directly with mix tailwind algora.