fix(app): preserve prior scrollRestoration value across mount/unmount

Address review feedback on the scoped scrollRestoration cleanup:
hard-setting 'auto' on unmount could clobber a non-default prior value
set elsewhere. Capture history.scrollRestoration on mount and restore
that exact value on unmount in both HomePage and PlotsPage.

Tests now cover both prior-state cases ('auto' and 'manual').

Author: claude

app/src/pages/HomePage.test.tsx

@@ -124,12 +124,20 @@ describe('HomePage', () => {
       history.scrollRestoration = original;
     });
 
-    it("sets scrollRestoration to 'manual' on mount and restores 'auto' on unmount", () => {
+    it("sets scrollRestoration to 'manual' on mount and restores the previous value on unmount", () => {
       history.scrollRestoration = 'auto';
       const { unmount } = render(<HomePage />);
       expect(history.scrollRestoration).toBe('manual');
       unmount();
       expect(history.scrollRestoration).toBe('auto');
     });
+
+    it('restores a non-default previous value on unmount instead of forcing auto', () => {
+      history.scrollRestoration = 'manual';
+      const { unmount } = render(<HomePage />);
+      expect(history.scrollRestoration).toBe('manual');
+      unmount();
+      expect(history.scrollRestoration).toBe('manual');
+    });
   });
 });

app/src/pages/HomePage.tsx

@@ -29,13 +29,15 @@ export function HomePage() {
   const { homeStateRef, saveScrollPosition } = useHomeState();
 
   // Disable browser's automatic scroll restoration so we can restore from
-  // our persisted state (homeStateRef.scrollY) instead. Restore on unmount
-  // so other routes get native back/forward behavior.
+  // our persisted state (homeStateRef.scrollY) instead. Capture the prior
+  // mode and restore it on unmount, so we don't clobber any non-default
+  // value set elsewhere and other routes get back native behavior.
   useEffect(() => {
     if (!('scrollRestoration' in history)) return;
+    const previous = history.scrollRestoration;
     history.scrollRestoration = 'manual';
     return () => {
-      history.scrollRestoration = 'auto';
+      history.scrollRestoration = previous;
     };
   }, []);
 

app/src/pages/PlotsPage.test.tsx

@@ -76,12 +76,20 @@ describe('PlotsPage', () => {
       history.scrollRestoration = original;
     });
 
-    it("sets scrollRestoration to 'manual' on mount and restores 'auto' on unmount", () => {
+    it("sets scrollRestoration to 'manual' on mount and restores the previous value on unmount", () => {
       history.scrollRestoration = 'auto';
       const { unmount } = render(<PlotsPage />);
       expect(history.scrollRestoration).toBe('manual');
       unmount();
       expect(history.scrollRestoration).toBe('auto');
     });
+
+    it('restores a non-default previous value on unmount instead of forcing auto', () => {
+      history.scrollRestoration = 'manual';
+      const { unmount } = render(<PlotsPage />);
+      expect(history.scrollRestoration).toBe('manual');
+      unmount();
+      expect(history.scrollRestoration).toBe('manual');
+    });
   });
 });

app/src/pages/PlotsPage.tsx

@@ -22,13 +22,15 @@ export function PlotsPage() {
   const { homeStateRef, saveScrollPosition } = useHomeState();
 
   // Disable browser's automatic scroll restoration so we can restore from
-  // our persisted state (homeStateRef.scrollY) instead. Restore on unmount
-  // so other routes get native back/forward behavior.
+  // our persisted state (homeStateRef.scrollY) instead. Capture the prior
+  // mode and restore it on unmount, so we don't clobber any non-default
+  // value set elsewhere and other routes get back native behavior.
   useEffect(() => {
     if (!('scrollRestoration' in history)) return;
+    const previous = history.scrollRestoration;
     history.scrollRestoration = 'manual';
     return () => {
-      history.scrollRestoration = 'auto';
+      history.scrollRestoration = previous;
     };
   }, []);