Add campaign editor shell

[Errordog2]

Summary

• add a campaign editor shell for deterministic demo campaign metadata, tags, status, preview, reset, and save actions
• add pure editor helpers for validation, empty-state selection, tag normalization, and snapshot creation
• cover snapshot normalization, required metadata validation, deterministic IDs, draftless empty states, and tag dedupe with focused tests

Closes #253

Validation

px prettier --check src/features/demo-admin-dashboard/campaignEditor.ts src/features/demo-admin-dashboard/components/CampaignEditorPanel.tsx src/features/demo-admin-dashboard/tests/campaignEditor.test.ts src/features/demo-admin-dashboard/index.ts

• git diff --check
• sensitive scan for account/payment/private-key markers returned no matches

px vitest run src/features/demo-admin-dashboard/tests/campaignEditor.test.ts could not start locally because this fresh checkout is missing the Vite config packages (@cloudflare/vite-plugin, @tailwindcss/vite, @tanstack/react-start/plugin/vite, @vitejs/plugin-react, �ite, �ite-tsconfig-paths)