Add campaign diff panel

[Errordog2]

Summary

• add a pure campaignDiff helper for comparing campaign snapshot metadata, tags, and draft inventories
• add a dashboard-only CampaignDiffPanel that surfaces added, removed, changed, and unchanged items
• cover identical snapshots, metadata edits, tag churn, and draft add/remove/change cases with focused tests

Closes #274

Validation

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

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

px vitest run src/features/demo-admin-dashboard/tests/campaignDiff.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)