Add cluster overview page

[Copilot]

Adds a Cluster Overview page under System in the sidebar. Fetches cluster info, data streams, and index metadata in parallel and displays them as summary cards.

Changes

• ClusterOverviewPage.tsx — New page component. Calls GET /, GET /_data_stream, and GET /_resolve/index/* concurrently. Shows cluster name/UUID/node, version/Lucene/build hash, and counts for data streams, indices, and aliases.
• AppSidebar.tsx — Added "Cluster Overview" nav item (with InfoIcon) under System, above Data Streams.
• useDashboardStore.ts — Extended currentPage union with "clusterOverview".
• App.tsx — Routing for the new page.
• ClusterOverviewPage.test.tsx — Component tests covering successful load, error state, and refresh.

Screenshot

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add cluster overview functionality</issue_title>
<issue_description></issue_description>

<agent_instructions>this is a little out of date but should get you going</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@strawgate /ai design the intended UX here please

Custom agent used: coding-assistant
AI coding assistant for the Elastic Peek repository

• Fixes Add cluster overview functionality #112

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.