Improve SFTP file view and upload handling

Add hidden-file toggle and search, prevent rapid-click path duplication, fix multipart upload headers, and raise backend upload size limits with clearer errors.

backend/src/main/java/com/sshmanager/exception/GlobalExceptionHandler.java

@@ -0,0 +1,42 @@
+package com.sshmanager.exception;
+
+import org.apache.tomcat.util.http.fileupload.impl.FileSizeLimitExceededException;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.RestControllerAdvice;
+import org.springframework.web.multipart.MaxUploadSizeExceededException;
+import org.springframework.web.multipart.MultipartException;
+
+import java.util.HashMap;
+import java.util.Map;
+
+@RestControllerAdvice
+public class GlobalExceptionHandler {
+
+    @ExceptionHandler(MaxUploadSizeExceededException.class)
+    public ResponseEntity<Map<String, String>> handleMaxUploadSize(MaxUploadSizeExceededException e) {
+        Map<String, String> err = new HashMap<>();
+        err.put("error", "上传失败：文件大小超过限制");
+        return ResponseEntity.status(HttpStatus.PAYLOAD_TOO_LARGE).body(err);
+    }
+
+    @ExceptionHandler(MultipartException.class)
+    public ResponseEntity<Map<String, String>> handleMultipart(MultipartException e) {
+        Throwable root = e;
+        while (root.getCause() != null && root.getCause() != root) {
+            root = root.getCause();
+        }
+
+        if (root instanceof FileSizeLimitExceededException
+            || (root.getMessage() != null && root.getMessage().contains("exceeds its maximum permitted size"))) {
+            Map<String, String> err = new HashMap<>();
+            err.put("error", "上传失败：文件大小超过限制");
+            return ResponseEntity.status(HttpStatus.PAYLOAD_TOO_LARGE).body(err);
+        }
+
+        Map<String, String> err = new HashMap<>();
+        err.put("error", "上传失败：表单解析异常");
+        return ResponseEntity.status(HttpStatus.BAD_REQUEST).body(err);
+    }
+}