Add SFTP toggle button to SSH tabs and enable fullscreen support

- Added 📁 folder icon button on SSH tabs to toggle the SFTP file browser
- Button appears on hover/active, styled consistently with the close button
- Removed conflicting Cmd+Shift+F shortcut (clashes with macOS find)
- Added platform-specific window options (Mac, Windows, Linux)
- Added F11 fullscreen toggle and Escape to exit fullscreen
- Cleaned up SFTP panel on session close

frontend/src/js/tabs.js

@@ -418,6 +418,20 @@ class TabManager {
             tab.classList.add('tab-pinned');
         }
 
+        // SFTP toggle button (SSH sessions only)
+        if (sessionInfo.protocol === 'ssh') {
+            const sftpBtn = createElement('button', 'tab-sftp', {
+                textContent: '📁',
+                title: 'Toggle SFTP file browser',
+                'aria-label': 'Toggle SFTP',
+            });
+            sftpBtn.addEventListener('click', (e) => {
+                e.stopPropagation();
+                this.app.sftp.toggle(sessionInfo.id);
+            });
+            tab.appendChild(sftpBtn);
+        }
+
         // Close button
         const closeBtn = createElement('button', 'tab-close', {
             textContent: '×',