Added dark theme to admin portal. Fixes #7

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.claude/rules/frontend.md

@@ -46,5 +46,7 @@ paths:
 
 - UI is the Bootstrap-based admin template in `assets/vendor/` plus Boxicons (`<i class="bx bx-...">`). Reuse its classes (`card`, `table`, `btn btn-primary`, `form-control`, `row`/`col`) before writing CSS.
 - Custom styles go to a CSS file in `assets/css/` and must be linked with `<Link rel="stylesheet">` in `src/components/header.rs` (order matters: after the vendor files to override them). `style/main.scss` is not linked anywhere and has no effect. Never edit `assets/vendor/` (the one app-owned file there is `assets/vendor/css/control.css`).
+- The admin portal has a light and a dark theme (`src/components/theme.rs`, switch in the navbar: auto / light / dark, stored in the `theme` cookie). Dark colours are overrides in `assets/css/dark.css`, which `AdminPortal` links with a `media` attribute — the public page and the login page never load it. When you add or restyle something in the admin portal, check it in both themes and add the dark override there; don't hard-code light colours (`bg-light`, `bg-white`, inline `color`) in admin views.
+- Charts are drawn on the server, so the chart server fns take a `dark` flag from `Theme::is_dark()`.
 - The public page banner style is `assets/data/banner.css`, rewritten at runtime by the appearance upload (`/*bg-img*/` marker) — keep that marker / the `background-image: url('...')` form intact.
 - Stylesheets and meta tags are declared in `src/components/header.rs`.