Make chat diagram creation resilient - #106

Summary

• add a per-answer Create diagram action so users can create a chart from the answer they are reading
• retry schema-mismatched diagram object generation once with a strict repair prompt
• return a friendly no-diagram state instead of surfacing raw schema validation failures when no valid chart can be produced

Validation

• pnpm exec vitest run src/components/chat-message-list.test.ts src/app/api/chat/diagram/route.test.ts src/domains/chat/diagram.test.ts src/components/chat-panel.test.ts --reporter=verbose
• pnpm exec tsc --noEmit --pretty false
• pnpm exec eslint src/domains/chat/diagram.ts src/domains/chat/diagram.test.ts src/components/chat-panel.tsx src/components/chat-panel.test.ts src/components/chat-message-list.tsx
• git diff --check
• pnpm build
• browser smoke: success and no-diagram paths both returned HTTP 200 and rendered the expected inline states