Add an export button to the ontology sidebar for the existing Turtle endpoint

[Schro-code]

Why

The server already exposes GET /api/v1/kbs/{id}/export?format=turtle but there is no UI to reach it — users must resort to curl or the browser address bar.

What changes

• web/src/pages/Ontology.tsx — adds a Download RailItem next to the existing Import button in the ontology sidebar, wired to window.open(/api/v1/kbs/{id}/export?format=turtle)
• web/src/i18n/en.ts — adds exportShort: "Export"
• web/src/i18n/zh.ts — adds exportShort: "导出"

How it was checked

• pnpm build and pnpm test pass locally
• Button appears in the ontology sidebar, clicking it opens the Turtle export in a new tab

Before review

• Every commit is signed off (git commit -s)
• For changes under web/: pnpm build and pnpm test pass
• UI strings are in both web/src/i18n/en.ts and zh.ts

[WaylandYang]

Thank you for this.

One change: GET /kbs/{id}/export exports the whole knowledge base (every fact with its provenance), not the ontology, so an "Export" next to the ontology's Import promises the wrong thing. Could the button say "Export knowledge base (RDF)" / "导出整个知识库（RDF）" and move to the base's settings (KbSettings)?

Thanks again.