feat: add SFTP drag-and-drop file handling and upload functionality

- Implemented drag-and-drop support for file uploads in the terminal.
- Added a modal for users to choose between uploading files or pasting contents.
- Enhanced SFTP browser with a directory picker for selecting upload destinations.
- Introduced methods for handling file uploads and reading file contents from the local system.
- Added a new API endpoint for uploading raw file data to the server.
- Implemented session management improvements to retrieve the current working directory for SSH sessions.
- Updated documentation to include an overview and quick start guide for Nexus Terminal.

frontend/src/js/sftp.js

@@ -158,6 +158,16 @@ class SFTPBrowser {
         }
     }
 
+    /**
+     * Returns the current directory for an open SFTP panel, or an empty string.
+     * @param {string} sessionId
+     * @returns {string}
+     */
+    getCwd(sessionId) {
+        var state = this.panels.get(sessionId);
+        return state ? state.cwd : '';
+    }
+
     /* ── Toolbar ──────────────────────────────────────────────────── */
 
     _buildToolbar(sessionId, state) {