feat: release ContentCloud v0.3.0

[wutongci]

Summary

• add an explicit platform-admin control plane for tenant and user operations
• add Agent Prompt workspace bootstrap with a public Markdown protocol and verified connection states
• split workspace, admin, authentication, and public pages into lazy React Router route trees
• migrate the npm installer to @limecloud/contentcloud and bump all release versions to 0.3.0
• secure session cookies behind HTTPS reverse proxies

Validation

• make check
• go test -race ./...
• pnpm --dir web test (15 tests)
• npm installer exercised against locally served signed assets for all supported release naming/checksum rules

Deployment

• no database migration files changed
• native systemd deployment only; Docker and ClamAV remain disabled
• production must set CONTENTCLOUD_PLATFORM_ADMIN_EMAILS before restart