feat(core): sub-page for setting panel (#11678)

**setting panel sub-page impl, with cascading pages support.**

## Usage
```tsx
// inside setting content
const island = useSubPageIsland();
const [open, setOpen] = useState(false);

if (!island) {
  return null;
}

return (
  <SubPageProvider
    island={island}
    open={open}
    onClose={() => setOpen(false)}
    backText="Back"
  />
);
```

### Preview

![CleanShot 2025-04-14 at 16.56.30.gif](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/LakojjjzZNf6ogjOVwKE/c042300d-c442-4708-a07a-54cd9f044abf.gif)

packages/frontend/core/src/modules/feature-flag/constant.ts

@@ -269,6 +269,13 @@ export const AFFINE_FLAGS = {
     configurable: isCanaryBuild,
     defaultState: isCanaryBuild,
   },
+  enable_setting_subpage_animation: {
+    category: 'affine',
+    displayName: 'Enable Setting Subpage Animation',
+    description: 'Apply animation for setting subpage open/close',
+    configurable: isCanaryBuild,
+    defaultState: false,
+  },
 } satisfies { [key in string]: FlagInfo };
 
 // oxlint-disable-next-line no-redeclare