refactor(api): unify /api contract across frontend, nginx, and backend

frontend/src/app/layouts/AppShellFooter.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useState } from "react";
 import { getBackendMeta } from "../../api";
 import { FRONTEND_VERSION } from "../../frontendVersion";
+import { readCachedBackendVersion, writeCachedBackendVersion } from "../../lib/backendVersionCache";
 
 function isAbortError(error: unknown): boolean {
   return (
@@ -10,7 +11,7 @@ function isAbortError(error: unknown): boolean {
 }
 
 export function AppShellFooter(): JSX.Element {
-  const [backendVersion, setBackendVersion] = useState<string | null>(null);
+  const [backendVersion, setBackendVersion] = useState<string | null>(() => readCachedBackendVersion());
 
   useEffect(() => {
     const ac = new AbortController();
@@ -20,13 +21,24 @@ export function AppShellFooter(): JSX.Element {
           return;
         }
         const v = meta.version;
-        setBackendVersion(typeof v === "string" && v.length > 0 ? v : "не указана");
+        const label = typeof v === "string" && v.length > 0 ? v : "не указана";
+        writeCachedBackendVersion(label);
+        setBackendVersion(label);
       })
       .catch((err) => {
         if (ac.signal.aborted || isAbortError(err)) {
           return;
         }
-        setBackendVersion("недоступна");
+        setBackendVersion((prev) => {
+          const cached = readCachedBackendVersion();
+          if (cached) {
+            return cached;
+          }
+          if (prev !== null) {
+            return prev;
+          }
+          return "недоступна";
+        });
       });
     return () => ac.abort();
   }, []);