fix(report): instant nav jump + scroll-position scrollspy (#958)

Скриншот-верификация PR #2104 с интеракциями вскрыла два бага навигации:

- Прыжок не работал: window.scrollTo({behavior:"smooth"}) — no-op на этой
  тяжёлой странице (Leaflet-карты / async §6 / lazy-чарты дают непрерывный
  reflow, отменяющий smooth-анимацию → страница оставалась на месте). Это и
  есть исходная жалоба «навигация не прыгает» (оригинал тоже был smooth).
  → behavior:"auto" (instant): надёжно, читается как настоящий прыжок.

- Scrollspy не трекал: узкая IntersectionObserver-полоса (-72px/-60%) пуста для
  нижней/высокой секции → if(next) держал старое значение (на §7 активной
  оставалась «1. Информация»). → scroll-position подход (rAF-throttled): активна
  последняя секция, чья верхушка прошла offset-линию; на низу страницы форсим
  последнюю секцию. Детерминированно, никогда не «пусто».

frontend/src/app/site-finder/analysis/[cad]/AnalysisPageContent.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useEffect, useRef, useState } from "react";
+import { useEffect, useState } from "react";
 import Link from "next/link";
 import { useQueryClient } from "@tanstack/react-query";
 
@@ -414,65 +414,65 @@ const TOP_LEVEL_IDS: string[] = NAV_GROUPS.flatMap((g) =>
 );
 
 // Sticky chrome offset (header + breadcrumb). The jump lands the section title
-// just below this; matches scrollMarginTop used on the section cards.
+// just below this; the scrollspy uses the same line to pick the active section.
 const SCROLL_OFFSET = 80;
 
-// Robust jump: bare scrollIntoView({behavior:"smooth"}) can land short when
-// async content (§6 forecast loads ~20-40s later) shifts heights mid-animation,
-// and it ignores the sticky offset. Compute an absolute target relative to the
-// document and scroll there, accounting for the sticky chrome.
+// Jump to a section. behavior:"auto" (INSTANT), NOT smooth: on this heavy report
+// (Leaflet maps, async §6 forecast, lazy charts) the continuous reflow silently
+// cancels a smooth window.scrollTo — it never moved («навигация не прыгает»).
+// Instant is reliable and reads as a real jump. Absolute document target +
+// SCROLL_OFFSET clears the sticky chrome and is robust to async height shifts.
 function scrollToSection(id: string) {
   const el = document.getElementById(id);
   if (!el) return;
   const top = el.getBoundingClientRect().top + window.scrollY - SCROLL_OFFSET;
-  window.scrollTo({ top, behavior: "smooth" });
+  window.scrollTo({ top, behavior: "auto" });
 }
 
 function AnalysisSidebarNav() {
-  // Scrollspy: highlight whichever top-level section is currently near the top
-  // of the viewport. Default to the first so something is lit before any scroll.
+  // Scrollspy: the active section is the LAST one whose top has scrolled past the
+  // offset line (deterministic, never "empty" — unlike a narrow IntersectionObserver
+  // band, which left the last/at-bottom section un-lit). Default to the first so
+  // something is lit before any scroll.
   const [activeId, setActiveId] = useState<string>(TOP_LEVEL_IDS[0]);
-  const observerRef = useRef<IntersectionObserver | null>(null);
 
   useEffect(() => {
-    // SSR / unsupported-env guard.
-    if (
-      typeof window === "undefined" ||
-      typeof IntersectionObserver === "undefined"
-    ) {
-      return;
-    }
+    if (typeof window === "undefined") return;
 
-    const targets = TOP_LEVEL_IDS.map((id) =>
-      document.getElementById(id),
-    ).filter((el): el is HTMLElement => el !== null);
-    if (targets.length === 0) return;
-
-    // Track the set of currently-intersecting sections; the active one is the
-    // first (topmost in scroll order) still inside the band. rootMargin pushes
-    // the band's top down past the sticky chrome and shrinks its bottom so the
-    // "active" section is the one near the top of the viewport — not whatever
-    // merely peeks in from below.
-    const visible = new Set<string>();
-
-    const observer = new IntersectionObserver(
-      (entries) => {
-        for (const entry of entries) {
-          if (entry.isIntersecting) visible.add(entry.target.id);
-          else visible.delete(entry.target.id);
+    let raf = 0;
+    const compute = () => {
+      raf = 0;
+      // At the page bottom the last section can be too short to reach the offset
+      // line — force it active so the last nav item lights up. (2px tolerance.)
+      const atBottom =
+        window.innerHeight + window.scrollY >=
+        document.documentElement.scrollHeight - 2;
+      if (atBottom) {
+        setActiveId(TOP_LEVEL_IDS[TOP_LEVEL_IDS.length - 1]);
+        return;
+      }
+      // Otherwise: the last section whose top has passed the offset line.
+      let current = TOP_LEVEL_IDS[0];
+      for (const id of TOP_LEVEL_IDS) {
+        const el = document.getElementById(id);
+        if (el && el.getBoundingClientRect().top - SCROLL_OFFSET <= 1) {
+          current = id;
         }
-        const next = TOP_LEVEL_IDS.find((id) => visible.has(id));
-        if (next) setActiveId(next);
-      },
-      { root: null, rootMargin: "-72px 0px -60% 0px", threshold: 0 },
-    );
+      }
+      setActiveId(current);
+    };
+    // rAF-throttle: at most one O(7) measurement per frame, never mid-frame jank.
+    const onScroll = () => {
+      if (raf) return;
+      raf = window.requestAnimationFrame(compute);
+    };
 
-    targets.forEach((el) => observer.observe(el));
-    observerRef.current = observer;
+    window.addEventListener("scroll", onScroll, { passive: true });
+    compute(); // initial state
 
     return () => {
-      observer.disconnect();
-      observerRef.current = null;
+      window.removeEventListener("scroll", onScroll);
+      if (raf) window.cancelAnimationFrame(raf);
     };
   }, []);