refactor: every project landing has its own theme

- landing.css is layout only and uses tokens (colors, fonts, hero, nav)
- palettes live in css/landing-<theme>.css; Robotex gets its own dark
  theme, MyPointVPN sets the same tokens
- theme defaults to the project folder name; the build fails if the
  theme file is missing, so a landing can't ship without styles

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

layouts/projects/single.html

@@ -6,11 +6,12 @@
 */ -}}
 {{- $p := .Params -}}
 {{- $home := cond (eq .Language.Lang hugo.Sites.Default.Language.Lang) "/" (printf "/%s/" .Language.Lang) -}}
-{{- $css := resources.Get "css/landing.css" -}}
-{{- /* params.theme adds css/landing-<theme>.css on top of the base styles */ -}}
-{{- with $p.theme }}{{ with resources.Get (printf "css/landing-%s.css" .) }}
-  {{- $css = slice $css . | resources.Concat (printf "css/landing-%s.bundle.css" $p.theme) }}
-{{- end }}{{ end -}}
+{{- /* Every landing has its own theme: css/landing-<theme>.css on top of the layout in
+  css/landing.css. The theme is params.theme or, by default, the project folder name. */ -}}
+{{- $theme := $p.theme | default .File.ContentBaseName -}}
+{{- $themeCSS := resources.Get (printf "css/landing-%s.css" $theme) -}}
+{{- if not $themeCSS }}{{ errorf "landing %q: theme file assets/css/landing-%s.css not found" .File.Path $theme }}{{ end -}}
+{{- $css := slice (resources.Get "css/landing.css") $themeCSS | resources.Concat (printf "css/landing-%s.bundle.css" $theme) -}}
 {{- $css = $css | minify | fingerprint -}}
 {{- $fonts := $p.fonts | default "family=Inter:wght@400;600;800&family=JetBrains+Mono:wght@400;600" -}}
 <!doctype html>
@@ -35,7 +36,7 @@
   <link href="https://fonts.googleapis.com/css2?{{ $fonts | safeURL }}&display=swap" rel="stylesheet">
   <link rel="stylesheet" href="{{ $css.RelPermalink }}" integrity="{{ $css.Data.Integrity }}">
 </head>
-<body{{ with $p.theme }} class="theme-{{ . }}"{{ end }}>
+<body class="theme-{{ $theme }}">
   <header class="nav">
     <div class="wrap nav__row">
       <a class="nav__logo" href="{{ $home }}">{{ .Title }}</a>