fix: prevent whiteboard data loss on Capture Screen and settings overwrite

- Save whiteboard on unmount (Capture Screen, resize) and visibility change
- Merge existing settings when saving App config to avoid overwriting whiteboard/teleprompter

Made-with: Cursor

Author: Funghi88

src/App.tsx

@@ -2396,7 +2396,9 @@ export default function App() {
 
   useEffect(() => {
     if (!settingsLoadedRef.current) return;
+    const current = loadSettings();
     saveSettings({
+      ...current,
       glowColor,
       pipPos,
       fullPagePipPos,

src/components/ExcalidrawBoard.tsx

@@ -414,11 +414,17 @@ export function ExcalidrawBoard({ onCanvasLayersChange, onWhiteboardTextureChang
 
   useEffect(() => {
     const onBeforeUnload = () => flushPersist();
+    const onVisibilityChange = () => {
+      if (document.visibilityState === "hidden") flushPersist();
+    };
     window.addEventListener("beforeunload", onBeforeUnload);
     window.addEventListener("pagehide", onBeforeUnload);
+    document.addEventListener("visibilitychange", onVisibilityChange);
     return () => {
       window.removeEventListener("beforeunload", onBeforeUnload);
       window.removeEventListener("pagehide", onBeforeUnload);
+      document.removeEventListener("visibilitychange", onVisibilityChange);
+      flushPersist(); // Save when unmounting (e.g. Capture Screen collapses whiteboard)
     };
   }, [flushPersist]);