feat: implement one-shot modifier keys for mobile terminal

- Redesign useSpecialKeys hook with modifier state tracking
- Add one-shot activation for Ctrl and Alt keys
- Visual feedback: active modifiers shown with yellow highlight
- Fix focusInput to use term.focus() instead of hidden input
- Always refocus terminal after sending any special key
- Add requestAnimationFrame for reliable focus restoration

apps/web/src/styles.css

@@ -3119,6 +3119,14 @@ a.nav-item,
   background: #1e5fa8;
 }
 
+.special-key-button.active-modifier {
+  background: #f5f543;
+  color: #1e1e1e;
+  border-color: #f5f543;
+  font-weight: 700;
+  box-shadow: 0 0 8px rgba(245, 245, 67, 0.5);
+}
+
 /* Special Keys Panel */
 .special-keys-panel-overlay {
   position: fixed;