Sync Pages branding and online workspace documentation

[wengzige]

What changed

• align the injected HTML editor chrome with the 帆页 Anchor Deck pine and warm-ivory palette
• replace the legacy blue interaction accents without changing editor layout or behavior
• integrate the online workspace into the existing README usage model instead of adding a detached section
• add regression coverage for the injected editor palette

Scope

The private GitHub Pages source is not included. This PR changes only the public README and shared injected editor runtime.

Validation

• npm run typecheck
• npm test — 277 tests passed
• GitHub Pages production build verified separately