fix(editor): prevent popMenu overflow on constrained viewports (#14827)

Fixes #14722

## Summary

`popMenu()` in
`blocksuite/affine/components/src/context-menu/menu-renderer.ts` uses
`autoPlacement` + `offset` in its default middleware, but no `shift()` —
so when `autoPlacement` picks a placement that would overflow the
viewport (e.g. database column menu opening near the top of a short
viewport), the menu stays overflowing and top items get clipped above
the viewport.

## Fix

Add `shift({ padding: 8 })` to the default middleware chain.

This matches the behavior of the sibling helper `createPopup()` in the
same file, which already includes `shift()` in its defaults.

## Reproducing (as reported in #14722)

Viewport ~879×461 (Chrome, macOS). Create a database block near the top
of the viewport → click a column name → menu opens upward and the top
items ("Rename", "Filter") get clipped above the viewport.



<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Enhanced context menu positioning on desktop to provide better
alignment and spacing adjustments near screen boundaries.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/components/src/context-menu/menu-renderer.ts

@@ -555,6 +555,7 @@ export const popMenu = (
         ],
       }),
       offset(4),
+      shift({ padding: 8 }),
     ],
     container: props.container,
     placement: props.placement,