Design: Add atmospheric background layers to docs shell

[hdcodedev]

Summary

Add atmospheric background layers and depth effects to the docs shell.

Why

Current background is mostly flat, while the design skill asks for depth/contextual visual detail.

Scope

• Introduce a subtle but intentional background system (mesh/noise/pattern/overlays).
• Keep content readability first; effect should not compete with docs text.
• Apply consistently to shell areas (main canvas/header/sidebar where appropriate).

Acceptance Criteria

• Body/main background is no longer flat-only.
• Visual layers are theme-aware.
• Performance remains acceptable (no heavy JS animation requirement).
• Accessibility/readability remains intact.

[hdcodedev]

Closed — addressed or superseded by modernization work in #44.