fix(workspace): preserve Base product content

[DR-Univer]

Summary

• remove the Base-specific DOM override that hid product-owned sidebar content
• remove the negative Canvas margin that clipped the Base summary bar by 36px
• keep Workspace ribbon, footer, and collaboration chrome disabled through supported plugin configuration
• update regression coverage to reject Base host CSS that changes product Canvas geometry

Validation

• pnpm --filter @univerjs/univer-workspace test (30 files, 139 tests)
• pnpm --filter @univerjs/univer-workspace exec vitest run test/unit/editor-integration-assets.test.ts (6 tests)
• pnpm --filter @univerjs/univer-workspace typecheck
• git diff --check origin/main...HEAD
• browser geometry: Base Canvas/content bottom = 719px, Workspace shell bottom = 720px, Canvas margin-bottom = 0px