feat(admin): navigation par sections sticky + support complet des themes

Navigation/ergonomie:
- barre sticky de pilules (Stats, Audit, Backups, Utilisateurs) sous le header
- scrollspy (IntersectionObserver) + scroll fluide au clic dans admin.js
- ancres + scroll-margin-top sur chaque section, layout responsive <600px

Themes:
- admin.html importe et appelle initThemes() (themes.js) au boot: la page
  reprend le theme/mode sauvegarde dans l'app (localStorage meme-origine)
- script anti-FOUC en <head> applique vite le mode avant le CSS

frontend/js/admin.js

@@ -399,6 +399,56 @@ function _wireAuditFilters() {
   }
 }
 
+// ── Section nav (scrollspy) ───────────────────────────────────────────
+
+/**
+ * Wire the sticky section nav: smooth-scroll on click + highlight the section
+ * currently in view (IntersectionObserver). Falls back to default anchor jump
+ * if the observer is unavailable.
+ */
+function _wireSectionNav() {
+  const links = Array.from(document.querySelectorAll("#admin-subnav a"));
+  if (!links.length) return;
+  const targets = links
+    .map((a) => document.getElementById((a.getAttribute("href") || "#").slice(1)))
+    .filter(Boolean);
+
+  // Smooth scroll on click
+  links.forEach((a) => {
+    a.addEventListener("click", (e) => {
+      const href = a.getAttribute("href");
+      if (!href || !href.startsWith("#")) return;
+      const t = document.getElementById(href.slice(1));
+      if (!t) return;
+      e.preventDefault();
+      t.scrollIntoView({ behavior: "smooth", block: "start" });
+    });
+  });
+
+  // Scrollspy: mark the section currently at the top of the viewport
+  let active = null;
+  const setActive = (id) => {
+    if (active === id) return;
+    active = id;
+    links.forEach((l) => l.classList.toggle("active", l.getAttribute("href") === "#" + id));
+  };
+
+  if ("IntersectionObserver" in window) {
+    const io = new IntersectionObserver(
+      (entries) => {
+        entries.forEach((en) => {
+          if (en.isIntersecting) setActive(en.target.id);
+        });
+      },
+      { rootMargin: "-64px 0px -70% 0px", threshold: 0 }
+    );
+    targets.forEach((t) => io.observe(t));
+  } else {
+    // Fallback: highlight the first section only (no scroll observation)
+    links[0] && links[0].classList.add("active");
+  }
+}
+
 // ── Auth gate ────────────────────────────────────────────────────────────
 
 /**
@@ -482,6 +532,7 @@ export async function init() {
   _populateAuditActions();
   _wireAuditFilters();
   _wireUserMgmt();
+  _wireSectionNav();
 
   // Initial loads (in parallel — none depends on another)
   loadStatsOnce();