Add Official Library source panel - #99

Summary

• consume the Knowhere demo catalog official_library payload and expose library categories/sources in initial workspace state
• add an Official Library section in the sources sidebar with ready rows, planned rows, and an Add action that materializes one selected demo source
• keep non-official legacy demo rows as normal source rows, and keep guest Add behavior routed through login

Verification

• pnpm exec vitest run src/integrations/knowhere-demo.test.ts src/domains/workspace/initial-state.test.ts src/components/sources-panel.test.ts src/components/workspace-source-workflow.test.ts src/components/workspace-shell-layout.test.ts
• pnpm exec vitest run src/domains/sources/route-service.test.ts 'src/app/api/sources/[sourceId]/route.test.ts'
• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/eslint (passes with existing warning in src/domains/sources/route-upload.ts)

Notes

• pnpm run lint is currently blocked locally by pnpm 11 verify-before-run build approval side effects; direct eslint passed without committing pnpm-workspace approval changes.
• This PR expects the API official_library payload from Add official library demo catalog knowhere#153.