v1.7.0: dual-pane SFTP file manager

Replace primitive upload/download form with a full dual-pane file manager
(local + remote) featuring directory browsing, navigation history,
file operations (upload, download, mkdir, delete, rename), progress
tracking, and persistent SFTP sessions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/ssh_client.py

@@ -308,5 +308,78 @@ class SSHClientWrapper:
         return f"Key generated: {self.key_path}"
 
 
+class SFTPSession:
+    """Persistent SFTP session for file browser."""
+
+    def __init__(self, server: dict, key_path: str):
+        self.server = server
+        self.key_path = key_path
+        self._client: paramiko.SSHClient | None = None
+        self._sftp: paramiko.SFTPClient | None = None
+
+    @property
+    def connected(self) -> bool:
+        try:
+            return (
+                self._client is not None
+                and self._sftp is not None
+                and self._client.get_transport() is not None
+                and self._client.get_transport().is_active()
+            )
+        except Exception:
+            return False
+
+    def connect(self):
+        self._client = _connect_client(self.server, self.key_path)
+        self._sftp = self._client.open_sftp()
+
+    def disconnect(self):
+        if self._sftp:
+            try:
+                self._sftp.close()
+            except Exception:
+                pass
+            self._sftp = None
+        if self._client:
+            try:
+                self._client.close()
+            except Exception:
+                pass
+            self._client = None
+
+    def listdir_attr(self, path: str) -> list:
+        return self._sftp.listdir_attr(path)
+
+    def stat(self, path: str):
+        return self._sftp.stat(path)
+
+    def mkdir(self, path: str):
+        self._sftp.mkdir(path)
+
+    def rmdir(self, path: str):
+        self._sftp.rmdir(path)
+
+    def remove(self, path: str):
+        self._sftp.remove(path)
+
+    def rename(self, old: str, new: str):
+        self._sftp.rename(old, new)
+
+    def upload(self, local_path: str, remote_path: str, progress_cb=None):
+        if progress_cb:
+            self._sftp.put(local_path, remote_path, callback=progress_cb)
+        else:
+            self._sftp.put(local_path, remote_path)
+
+    def download(self, remote_path: str, local_path: str, progress_cb=None):
+        if progress_cb:
+            self._sftp.get(remote_path, local_path, callback=progress_cb)
+        else:
+            self._sftp.get(remote_path, local_path)
+
+    def normalize(self, path: str) -> str:
+        return self._sftp.normalize(path)
+
+
 def _shell_quote(s: str) -> str:
     return "'" + s.replace("'", "'\\''") + "'"