🎨 Palette: Improve ecosystem dropdown accessibility

[divineforge]

💡 What: Removed role="menu" and role="menuitem" from the ecosystem navigation dropdown, and added aria-hidden="true" to the decorative dropdown arrow.
🎯 Why: The dropdown relies on CSS hover states rather than full JavaScript keyboard navigation (e.g., arrow keys). Applying role="menu" without this expected interaction model traps screen reader users or confuses them, as they expect a complex desktop-like menu. Standard semantic HTML links within a navigation landmark provide a more robust and predictable experience. Additionally, hiding the decorative arrow prevents useless screen reader noise.
♿ Accessibility: Improves screen reader navigation flow through the site header by presenting the dropdown items as standard, predictable links rather than a broken application menu.

---

PR created automatically by Jules for task 2715561526349424621 started by @divineforge

[cloudflare-workers-and-pages]

Deploying gzen with    Cloudflare Pages

Latest commit:	6e8fb75
Status:	🚫  Build failed.

View logs

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.