menu: Fix render nested submenus with increasing paint priority (#2587)

## Description

Nested submenus were drawn as plain `anchored()` elements that shared
the same paint order, so shallow-level content (e.g. the underlying list
behind the menu) could show through deeper submenu panels.

This change gives each `PopupMenu` a `priority: usize` field (top-level
= 1) that increments by one for every nested level, and wraps both the
menu body and its submenu panels in
`deferred().with_priority(priority)`:

- `PopupMenu::render()` now returns
`deferred(menu).with_priority(self.priority)` so the top-level menu is
itself a deferred layer.
- Each submenu panel is rendered with `.with_priority(self.priority +
1)`.
- `submenu_with_icon` propagates `priority = parent_priority + 1` down
the tree.

The result is a strict paint order: top-level (1) < submenu (2) <
sub-submenu (3) < ..., so deeper menus always paint on top of shallower
ones.

## Screenshot

| Before                       | After                       |
| ---------------------------- | --------------------------- |
| [<img width="500" height="163" alt="before"
src="https://github.com/user-attachments/assets/0da6b4c2-b26c-4cb8-9e19-ef77e362bcd0"
/>] | [<img width="500" height="163" alt="after"
src="https://github.com/user-attachments/assets/55f3dd49-f51d-4fb8-8d23-7b658f08bf43"
/>] |

## How to Test

1. To reproduce the multi-level paint-order bug, a menu with **at least
two nested submenu levels** is required (e.g. a right-click tree where
each item opens a submenu that itself contains another submenu). Expand
several levels so deeper panels overlap shallower ones.
2. **Before:** shallow-level text was visible through deeper submenu
panels (see the *Before* screenshot).
3. **After:** deeper submenus always paint on top, so background text no
longer bleeds through (see the *After* screenshot).

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [x] Passed `cargo run` for story tests related to the changes.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)

---------

Co-authored-by: Lumen <haifeng_dai_cn@163.com>
Co-authored-by: Jason Lee <huacnlee@gmail.com>