feat: implement manual save via Ctrl+S with toast and increase autosave interval

- Increased autosave interval to 30s.
- Captured Ctrl+S / Meta+S to trigger manual save.
- Added a minimalistic, translucent toast in the top right to display saving status.

src/App.tsx

@@ -143,7 +143,7 @@ export function App() {
       .then((body) => {
         setDrawings((current) => replaceMeta(current, body.drawing));
         if (isManual) {
-          setToastMessage("Saved.");
+          setToastMessage("Saved");
           if (toastTimeoutRef.current !== null) {
             clearTimeout(toastTimeoutRef.current);
           }
@@ -153,7 +153,7 @@ export function App() {
       .catch((saveError: unknown) => {
         setError(saveError instanceof Error ? saveError.message : "Save failed");
         if (isManual) {
-          setToastMessage("Save failed.");
+          setToastMessage("Save failed");
           if (toastTimeoutRef.current !== null) {
             clearTimeout(toastTimeoutRef.current);
           }