fix: wire export shortcut, text tool canvas integration, and undo rendering

1. Export dialog (Ctrl+Shift+S): replaced react-hotkeys-hook handler with
   a capture-phase keydown listener on window so the browser's native
   "Save Page As" dialog is intercepted before it can fire.

2. Text tool: created useTextTool hook that spawns an inline textarea
   overlay on canvas click, commits the text as a Konva Text object on
   blur/Enter, and wired it into the useActiveToolHandlers dispatcher.

3. Undo (Ctrl+Z): removed the 500ms debounce from zundo's handleSet.
   The debounce caused a race where calling undo before the timer fired
   would discard the future-states stack, making undo appear to do
   nothing.  The equality function (keyed on _historyVersion) already
   prevents intermediate states from being recorded, so the debounce
   was redundant.

apps/web/src/hooks/use-editor-shortcuts.ts

@@ -357,14 +357,23 @@ export function useEditorShortcuts(callbacks?: { onSave?: () => void; onExport?:
   );
 
   // Ctrl+Shift+S / Cmd+Shift+S - Export image
-  useHotkeys(
-    "mod+shift+s",
-    (e) => {
-      e.preventDefault();
-      callbacks?.onExport?.();
-    },
-    { preventDefault: true },
-  );
+  // Use a raw keydown listener at capture phase to reliably prevent the
+  // browser's "Save Page As" dialog, which fires before react-hotkeys-hook
+  // can intercept.
+  const onExportRef = useRef(callbacks?.onExport);
+  onExportRef.current = callbacks?.onExport;
+
+  useEffect(() => {
+    const handler = (e: KeyboardEvent) => {
+      if ((e.metaKey || e.ctrlKey) && e.shiftKey && e.key.toLowerCase() === "s") {
+        e.preventDefault();
+        e.stopPropagation();
+        onExportRef.current?.();
+      }
+    };
+    window.addEventListener("keydown", handler, { capture: true });
+    return () => window.removeEventListener("keydown", handler, { capture: true });
+  }, []);
 
   // Ctrl+A / Cmd+A - Select all
   useHotkeys(