feat: add shared assets workspace and studio history

[erickkkyt]

Summary

• add a project-scoped Assets workspace shared with Studio and Canvas
• add a persistent Studio generation feed, preview details, and full model parameter controls
• preserve full project names, show the latest 80 generations in chronological order, and keep local demo media out of Git
• synchronize English/Chinese copy and public architecture documentation

Verification

• pnpm build
• pnpm typecheck
• pnpm test: 218 passed, 0 failed
• pnpm i18n:check: 543 English / 543 Chinese keys
• pnpm audit --prod --audit-level high: no known vulnerabilities
• staged security scan: 45 feature files and 5 documentation files, no findings

Review

• pre-landing code review: clean
• design review lite: clean
• no authentication, billing, payment, credits, or SaaS modules added

Local-only exclusions

• public/demo-assets/ remains local and is ignored
• local databases, provider keys, build output, and generated route/i18n files are not included

Documentation

• README.md and README.zh-CN.md: added the project-scoped Assets view and updated data flow
• ARCHITECTURE.md and WORKSPACE_MODES.md: documented Studio, Canvas, and Assets as three shared project views
• DESIGN.md and AGENTS.md: aligned local-first product boundaries and cold-blue Canvas semantics