Cordalo error-page previews

Forced-theme review pages; production assets use the browser color-scheme preference.

• Shared logo asset catalog
• Every server error message — light and dark previews
• 503 — light · dark
• 500 fallback — light · dark
• 1xxx — light · dark
• WAF block — light · dark
• IP block — light · dark
• Rate limit — light · dark
• Challenge — light · dark
• Optional 404 — light · dark