feat: add preview container environment controls

[numnx]

Summary

• Add preview container environment variable controls in the Browser right sidebar and Settings defaults.
• Persist per-container preview env overrides and merge them with project defaults when starting preview containers.
• Validate preview env names and reserved runtime variables before writing Docker env files.
• Rename the built-in styleguide template display name to Code UX.
• Update canonical and public docs for browser preview environment behavior.

Why

Preview containers sometimes need explicit app-level environment variables for bind/security flags or framework-specific preview behavior. This lets users configure those values from the dashboard instead of editing preview scripts.

Validation

pnpm run lint
pnpm run test:backend -- tests/backend/repositories/settings-sanitizer.test.ts tests/backend/repositories/sprint-preview-repository.test.ts tests/backend/server/preview-routes.test.ts tests/backend/services/sprint-preview-service-unit.test.ts
pnpm run test:dashboard -- tests/dashboard/v2/browser-page.test.tsx
pnpm run test:backend -- tests/backend/domain/settings/design-guidance-catalog.test.ts tests/backend/domain/projects/project-initializer.test.ts
pnpm run test:dashboard -- tests/dashboard/lib/settings-view-models.test.ts tests/dashboard/v2/top-nav-selectors.test.tsx
pnpm run build

Additional local check: /health returned UP at http://127.0.0.1:4444/health.

Risk / Rollback

Low-to-moderate risk: this adds an optional nullable DB column and additive preview env handling. Roll back by reverting the commit; existing preview sessions remain usable because missing overrides default to an empty set.