[Feature] Theme system — install, develop, preview, publish, and operate

[gaoredu]

Is your feature request related to a problem? Please describe.

ReactPress aims to be WordPress-like for themes, but the platform stack is incomplete on HEAD:

• themes/package.json calls reactpress theme add, but CLI has no theme command; server has no extension module
• Toolkit defines preview API, 60s ISR, and ports 3003–3008 — server/CLI are not wired
• Themes can be copied into the repo but cannot be installed, upgraded, previewed, or kept fresh after publish like a real theme platform

Describe the solution you'd like

One shared contract in toolkit; thin server/CLI wrappers. Phased delivery:

P0 — Core

• Server extension module: install, activate, preview-draft API
• CLI: theme add|list|update|remove + themes.lock.json + runtime at .reactpress/runtime/{id}/
• Move theme-registry into toolkit (server/CLI import shared code, no require('../cli/lib/...'))
• On publish / update / delete / scheduled publish → on-demand Next.js revalidation; toolkit path helpers + HMAC /api/revalidate in starter

P1 — Developer & discovery

• reactpress theme dev <id> — symlink themes/{id} in dev, inject REACTPRESS_API_URL
• Multi-slot preview pool (3003–3008), hot-swap draft tokens
• Hosted catalog.json, theme search, admin Discover tab

P2 — Production

• Zero-downtime activation: build → health check → atomic active-theme.json; theme rollback

reactpress theme add <spec>
reactpress theme list
reactpress theme update [--check] [id]
reactpress theme remove <id>
reactpress theme dev <id>
reactpress theme rollback

Describe alternatives you've considered

• Manual runtime copy — no lock file, no admin, breaks on every edit
• Webhooks-only cache refresh — every site builds a sidecar; does not fix 60s ISR for editors
• Duplicate registry in server and CLI — drifts; breaks standalone API deploy
• Single preview port — works but slow; iframe blanks on every theme switch
• PM2 hard restart for prod switch — current behavior; risks 502 with no rollback