Simplify search UI and add saved searches to shelf

[disnet]

Summary

• Simplify expanded search header by removing expand all, collapse all, and close buttons; enhance save button with "Save Search" text
• Update search header styling to remove border separator and align controls with results content
• Add saved searches as first-class shelf items so users can save searches, add them to shelf, and view results in the shelf panel
• Enable add-to-shelf for saved searches via toolbar, keyboard shortcuts, and context menu
• Improve context menu by adding transparent backdrop to close menu when clicking outside

Test plan

• Open a search and verify the simplified header with just the save button
• Save a search and verify it shows the "Saved" indicator
• Add a saved search to the shelf via the toolbar or context menu
• Verify the shelf item displays the full search interface with results
• Right-click on a saved search in the sidebar and verify shelf options work
• Click outside the context menu and verify it closes

[cloudflare-workers-and-pages]

Deploying flint-website with    Cloudflare Pages

Latest commit:	d445767
Status:	✅  Deploy successful!
Preview URL:	https://daa39fa6.flint-website.pages.dev
Branch Preview URL:	https://disnet-search-save-button.flint-website.pages.dev

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