feat(site-finder): deep-link tabs via ?tab=env|land|market URL param

Tab state now syncs with URL — можно поделиться ссылкой на конкретный
таб (например /site-finder?tab=market для тренда цен по последнему cad).

- useRouter + useSearchParams (Next 15 app router, client-component)
- router.replace (не push) чтобы tab switches не засоряли history
- useEffect on searchParams → back/forward работают
- overview = default, не пишется в URL (clean)
- Type-guard isTabId(): только valid values из URL переключают state

frontend/src/app/site-finder/page.tsx

@@ -1,8 +1,9 @@
 "use client";
 
-import { useState } from "react";
+import { useEffect, useState } from "react";
 import dynamic from "next/dynamic";
 import Link from "next/link";
+import { useRouter, useSearchParams } from "next/navigation";
 import type { FeatureCollection } from "geojson";
 
 import { CadInput } from "@/components/site-finder/CadInput";
@@ -47,6 +48,12 @@ const TABS: Array<{ id: TabId; label: string }> = [
   { id: "market", label: "Рынок" },
 ];
 
+const TAB_IDS = TABS.map((t) => t.id);
+
+function isTabId(v: string | null | undefined): v is TabId {
+  return !!v && (TAB_IDS as readonly string[]).includes(v);
+}
+
 // ── KPI helpers ───────────────────────────────────────────────────────────────
 
 type KpiColor = "neutral" | "amber" | "green" | "red" | "blue";
@@ -74,11 +81,39 @@ function scoreKpiColor(score: number, max: number): KpiColor {
 // ── Page ──────────────────────────────────────────────────────────────────────
 
 export default function SiteFinderPage() {
+  const router = useRouter();
+  const searchParams = useSearchParams();
+  const initialTab: TabId = (() => {
+    const t = searchParams.get("tab");
+    return isTabId(t) ? t : "overview";
+  })();
+
   const { mutate, data, isPending, error, isIdle } = useSiteAnalysis();
   const [isochrones, setIsochrones] = useState<FeatureCollection | undefined>(
     undefined,
   );
-  const [tab, setTab] = useState<TabId>("overview");
+  const [tab, setTabState] = useState<TabId>(initialTab);
+
+  // Sync tab → URL query (?tab=env). overview = default, не пишем в URL.
+  function setTab(next: TabId) {
+    setTabState(next);
+    const params = new URLSearchParams(searchParams.toString());
+    if (next === "overview") {
+      params.delete("tab");
+    } else {
+      params.set("tab", next);
+    }
+    const qs = params.toString();
+    router.replace(qs ? `?${qs}` : "?", { scroll: false });
+  }
+
+  // Listen to back/forward navigation — reflect URL tab in state
+  useEffect(() => {
+    const urlTab = searchParams.get("tab");
+    const validTab: TabId = isTabId(urlTab) ? urlTab : "overview";
+    if (validTab !== tab) setTabState(validTab);
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [searchParams]);
 
   function handleAnalyze(cadNum: string) {
     setIsochrones(undefined);