Implement global search

[songJianwen123324]

Closes #3

Summary

• Adds a global search box to the authenticated layout
• Searches navigation pages, items, and admin-only users
• Lets users open the first result with Enter or click any result

Checks

• npm run build
• npx biome check --no-errors-on-unmatched --files-ignore-unknown=true src/components/Common/GlobalSearch.tsx src/routes/_layout.tsx src/components/Common/UserMenu.tsx src/components/Common/Navbar.tsx

[songJianwen123324]

Hi! I opened this PR for #3, which has the $5 bounty label. It adds global search across pages, items, and admin-only users, and the build/biome checks pass locally. Please let me know if you would like any changes.