Work on download files(private chats and groups)

src/main/java/org/to/telegramfinalproject/Client/DownloadIndexRegistry.java

@@ -0,0 +1,30 @@
+// DownloadIndexRegistry.java
+package org.to.telegramfinalproject.Client;
+
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+
+public final class DownloadIndexRegistry {
+    private static final ConcurrentHashMap<UUID, DownloadsIndex> INSTANCES = new ConcurrentHashMap<>();
+    private static volatile boolean HOOK_REGISTERED = false;
+
+    private DownloadIndexRegistry() {}
+
+    public static DownloadsIndex forAccount(UUID accountId) {
+        registerHookOnce();
+        return INSTANCES.computeIfAbsent(accountId, DownloadsIndex::new);
+    }
+
+    public static void closeAccount(UUID accountId) {
+        DownloadsIndex idx = INSTANCES.remove(accountId);
+        if (idx != null) idx.saveQuietly();
+    }
+
+    private static synchronized void registerHookOnce() {
+        if (HOOK_REGISTERED) return;
+        Runtime.getRuntime().addShutdownHook(new Thread(() -> {
+            for (DownloadsIndex idx : INSTANCES.values()) idx.saveQuietly();
+        }));
+        HOOK_REGISTERED = true;
+    }
+}