fix(nav-icon): remove white border in dark mode popover

[fi3ework]

UI broken on https://rspack.rs/ and all other rstack sites.

• Fix Ant Design 6 popover white border in dark mode by resetting .ant-popover-container styles
  • Ant Design 6 introduces a new .ant-popover-container layer, which has a default white background and shadow.
  • In dark mode, this white background creates a visual problem of a "white border."
    • Our fix:
      • 1. Set background-color: transparent; box-shadow: none; on .ant-popover-container to remove the white background and shadow.
      • 2. Manually add the shadow back to .ant-popover-content to maintain the visual effect. Previously, it was not necessary to manually write box-shadow because Ant Design 5 did not have this extra container layer, and the shadow was directly on the content layer.
• Add Chromatic CI workflow for visual regression testing
• Add @storybook/test for interaction testing with play functions
• Add expanded state stories (NavIconStoryExpanded, NavIconStoryExpandedDark) with click simulation
• Add AGENTS.md with project coding guidelines

now visual diff works on Chromatic https://www.chromatic.com/test?appId=696728d1bd37a962a7f22c4b&id=6967359abb77a1e9f9b073ee to prevent regression.

[chenjiahan]

Cool 👍