feat: ship Beta 7 creator quality and theme system

[xuhuanstudio]

What changed

• unify local previews and injected Codex styling around one semantic theme contract covering surfaces, text, icons, borders, status and diff colors, typography, materials, depth, and motion
• replace invented preview layouts with a shared Codex shell and responsive master-detail resource workspaces
• add companion alignment, global scaling, safe placement modes, behavior audition, compiler readiness, and edge-quality review
• complete Beta 7 release metadata, localized update notes, compatibility disclosures, and download links
• harden compact theme and companion details, home compatibility copy, persistent setup-bar clearance, and visual regression coverage

Verification

• 271 desktop unit and component tests
• 22 theme-core validation tests
• 20 Rust tests, rustfmt, and Clippy
• 18 manager E2E flows and paired screenshot baselines
• desktop and site production builds
• JavaScript high-severity dependency audit and cargo audit
• release version synchronization, schema examples, and local link checks
• ad-hoc signed Apple Silicon application bundle verification and launch smoke test

Release boundary

This remains an unsigned Creator Beta. Windows artifacts are CI-built and structurally verified; independent Windows 11 lifecycle evidence remains required before RC.