feat: add latency indicator and broadcast target selection

- Added a latency indicator to the status bar, displaying round-trip time for SSH sessions.
- Implemented functionality to select specific sessions for broadcasting input, allowing users to manage broadcast targets.
- Enhanced SFTP functionality with bookmark management, enabling users to bookmark directories and navigate to them easily.
- Introduced a modal for user-defined snippet variables, allowing dynamic command execution with placeholders.
- Added port forwarding management for SSH sessions, including the ability to start and stop port forwards.
- Improved terminal output export functionality, allowing users to download or copy terminal output to clipboard.

frontend/src/js/state.js

@@ -28,6 +28,9 @@ class State {
         /** @type {boolean} Whether broadcast mode is active (keystrokes to all sessions). */
         this.broadcastMode = false;
 
+        /** @type {Set<string>|null} Session IDs to broadcast to. Null means all. */
+        this.broadcastTargets = null;
+
         /** @type {Map<string, Set<Function>>} event name → callbacks */
         this._listeners = new Map();
     }