fix: mermaid diagrams don't respect dark mode theme #127

Problem

`src/components/mdx/mermaid.tsx` initializes mermaid with `theme: 'default'` (line 19), which always renders light-mode diagrams regardless of the site's current theme. In dark mode, diagrams appear as bright white boxes.

Suggested Fix

• Read the current theme from the document (`document.documentElement.dataset.theme` or the Apsara ThemeProvider context)
• Initialize mermaid with `theme: 'dark'` when in dark mode
• Re-render diagrams on theme change (listen for `data-theme` attribute mutations)