Add dark mode support via prefers-color-scheme

[bonheur15]

Problem

The landing page has no dark mode support. The CSS custom property architecture (--ink, --paper, --muted, --line, --accent) makes adding dark mode straightforward, but it hasn't been implemented.

Location: CSS in internal/web/views/layout.templ (lines 16-22 define the variables, lines 31-35 swap some for the Image CDN tab)

Current State

Light theme only. The theme swap at lines 31-35 only overrides --paper and --accent when the Image CDN tab is active — not --ink, --muted, or --line.

Proposed Fix

Add a prefers-color-scheme: dark media query that overrides the CSS custom properties:

@media (prefers-color-scheme: dark) {
    :root {
        --ink: #e4e8eb;
        --muted: #8b949e;
        --line: #30363d;
        --paper: #0d1117;
        --accent: #58a6ff;
    }
    
    .builder {
        background: #161b22;
    }
    
    code, pre, .builder .out {
        background: #0d1117;
    }
    
    .badge {
        background: #1f2937;
        color: #58a6ff;
    }
}

Why This Matters

Dark mode reduces eye strain in low-light environments and is an expected feature for developer tools. The existing CSS variable architecture means this can be added with minimal changes — just the media query block above.