feat: add prefers-color-scheme light mode support to website

[h3qing]

What

The marketing website currently only supports dark mode. Add light mode support that respects `prefers-color-scheme`.

Current state

• `website/index.html` uses CSS custom properties (`--bg`, `--text`, etc.) defined in `:root`
• All colors are dark theme (dark backgrounds, light text)
• No `@media (prefers-color-scheme: light)` rules exist

Approach

1. Add a `@media (prefers-color-scheme: light)` block in the `<style>` section
2. Override the CSS custom properties with light-mode values
3. Ensure the Mando SVGs (which have hardcoded dark outlines) still look good on light backgrounds

Example light palette:
```css
@media (prefers-color-scheme: light) {
:root {
--bg: #FAFAF8;
--surface: #FFFFFF;
--border: #E5E0DA;
--text: #1A1612;
--text-dim: #5C5248;
--text-muted: #8A7A6A;
/* ... etc */
}
}
```

Files to modify

• `website/index.html` — add light mode CSS variables + any element-specific overrides

Acceptance criteria

• Website auto-switches to light mode based on system preference
• All text is readable on light backgrounds
• Mando SVGs look good on light backgrounds
• Buttons, cards, and hover states work in both modes
• No hardcoded colors outside of CSS variables (or overridden where needed)