Add corporate and nitro themes, inline theme banners, code palette sets

- Two new themes (corporate light/dark, nitro racing) and a purple
  retune; theme banner SVGs moved to pagerite/themes/, inlined by the
  backend, recolored per scheme via CSS classes, with scroll parallax.
- pygments.css maps tokens onto --code-* variables; light and dark
  palette sets in the base stylesheet resolve via light-dark() from the
  theme's color-scheme; themes just pick a set or retint --code-bg.
- Sidebar omitted unless the section has at least two published items;
  fetch-navigation and the site editor handle it appearing/disappearing.
- Floated figures keep captions below at 30% of the text column; h1/h2
  clear floats so images never overflow into the next section.
- Links use --link (color-mix of text and accent), weight 500, no
  underlines; hover goes full accent.

frontend/src/SiteEditor.vue

@@ -105,11 +105,22 @@ function openPath(p) {
 
 // --- In-place navigation (no transitions, replaceState) ------------------
 function swapRegions(doc) {
-  for (const id of ['page-banner', 'nav', 'sidebar', 'main']) {
+  for (const id of ['page-banner', 'nav', 'main']) {
     const fresh = doc.getElementById(id)
     const el = document.getElementById(id)
     if (fresh && el) el.replaceWith(document.importNode(fresh, true))
   }
+  // #sidebar is omitted entirely when the section has no sub-navigation,
+  // so it may be absent on either side: replace, insert, or remove.
+  const freshSidebar = doc.getElementById('sidebar')
+  const curSidebar = document.getElementById('sidebar')
+  if (freshSidebar && curSidebar) {
+    curSidebar.replaceWith(document.importNode(freshSidebar, true))
+  } else if (freshSidebar) {
+    document.getElementById('main')?.before(document.importNode(freshSidebar, true))
+  } else if (curSidebar) {
+    curSidebar.remove()
+  }
   // The brand link lives in the header, outside the swappable regions,
   // and is absent entirely when no brand is configured.
   const freshBrand = doc.getElementById('brand')
@@ -283,6 +294,8 @@ const theme = ref('purple')
 const THEME_OPTIONS = [
   { value: '', label: 'none' },
   { value: 'purple', label: 'purple' },
+  { value: 'corporate', label: 'corporate' },
+  { value: 'nitro', label: 'nitro' },
 ]
 
 async function loadSettings() {