css: move sidebar/TOC component styles out of layout, replace <small> with <h3>

- sidebar and TOC typography/colors moved from 08-layout.css to 03-components.css
- nav[data-nav="sidebar"] and aside[data-toc] layout-only rules remain in 08-layout.css
- <small> section labels replaced with <h3> in sidebar and TOC (semantic + accessible)
- dead selectors removed: nav[data-nav="sidebar"] nav a → nav[data-nav="sidebar"] a,
  nav[data-nav="toc"] (Hugo never outputs data-nav="toc")
- webkit scrollbar pseudo-elements removed (scrollbar-width/color sufficient)
- sidebar/TOC sticky top values split: sidebar top space-4, TOC top space-8 + padding-top
- max-height magic number (--size-px-10) replaced with token expression
- layer convention established: 03=component identity, 04=modifiers, 08=placement only

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

layouts/docs/single.html

@@ -19,7 +19,7 @@
       {{- if $menu -}}
         {{- range $menu -}}
           {{- if .HasChildren -}}
-          <small>{{ .Name }}</small>
+          <h3>{{ .Name }}</h3>
           <ul>
             {{- range .Children -}}
             <li>
@@ -43,7 +43,7 @@
         {{- end -}}
       {{- else -}}
         {{- with .CurrentSection -}}
-        <small>{{ .Title }}</small>
+        <h3>{{ .Title }}</h3>
         <ul>
           {{- range .RegularPages -}}
           <li>
@@ -81,7 +81,7 @@
 
     {{- with .TableOfContents -}}
     <aside data-toc>
-      <small>On this page</small>
+      <h3>On this page</h3>
       {{ . }}
     </aside>
     {{- end -}}