feat: search items from dashboard

[zhangzhenghao]

Summary

• Update the Items page search box to call GET /api/items?q=... instead of exact GET /api/item/{id} lookup
• Show backend search errors inline, including the unsupported-search response when search columns are not configured
• Disable cursor pagination controls while search results are displayed; clearing the search input resets to normal item pagination

Dependency

• Requires gorse core PR: support search items on dashboard gorse#1282

Test Plan

• pnpm exec eslint src/views/Items.vue
• pnpm lint
• pnpm build
• git diff --check