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/css/tabs.css

@@ -128,6 +128,41 @@
 .tab-status.is-error        { background: var(--error); }
 .tab-status.is-disconnected { background: var(--text-tertiary); }
 
+/* -----------------------------------------------------------------------
+   SFTP toggle button — revealed on hover (SSH tabs only)
+   ----------------------------------------------------------------------- */
+.tab-sftp {
+    width: 16px;
+    height: 16px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    border-radius: var(--radius-sm);
+    font-size: 10px;
+    color: var(--text-tertiary);
+    flex-shrink: 0;
+    opacity: 0;
+    transition:
+        opacity var(--transition-fast),
+        background var(--transition-fast),
+        color var(--transition-fast);
+    cursor: pointer;
+    background: none;
+    border: none;
+    padding: 0;
+    margin-left: auto;
+}
+
+.tab:hover .tab-sftp,
+.tab.active .tab-sftp {
+    opacity: 1;
+}
+
+.tab-sftp:hover {
+    background: var(--bg-active);
+    color: var(--accent);
+}
+
 /* -----------------------------------------------------------------------
    Close button — revealed on hover
    ----------------------------------------------------------------------- */
@@ -147,7 +182,7 @@
         background var(--transition-fast),
         color var(--transition-fast);
     cursor: pointer;
-    margin-left: auto;
+    margin-left: 2px;
 }
 
 .tab:hover .tab-close,