feat: add 16688 Studio provider and deployment profile

[liupingsong89-creator]

Summary

• add opt-in 16688 Studio branding and build mode while preserving BeatAPI defaults
• add a 16688 AI image-provider prototype with NewAPI and configurable Gemini media-gateway transports
• add a hardened Docker/nginx deployment profile for studio.16688.org
• improve Windows test portability, Canvas empty-state persistence, Studio localization, and narrow-screen layout

Scope

• 11 commits and 54 changed files
• text-to-image only for the 16688 provider prototype; reference-image editing and video remain outside this provider stage
• credentials remain server-side and encrypted in local SQLite

Verification

• pnpm typecheck
• pnpm test (428 passing)
• pnpm i18n:check (632 keys in English, Chinese, and Japanese)
• pnpm build
• pnpm build:16688
• local browser smoke test, including 390px and 648px overflow checks
• MCP stdio handshake covered by the test suite
• credentialed/paid provider generation was not run

[liupingsong89-creator]

UI screenshots

Captured locally in the 16688 build mode with no API credentials entered. Related proposal: #19.

16688 Studio home

Studio workspace

16688 AI provider configuration