Design: Redesign 404 page to match docs design system #42

Summary

Redesign the 404 page to match docs visual direction and systemized styling.

Why

Current 404 page is heavily inline-styled and stylistically detached from the rest of the docs shell.

Scope

• Build 404 layout using shared classes/tokens.
• Align typography, spacing, color, and CTA treatment with docs design system.
• Ensure mobile and desktop polish.

Acceptance Criteria

• 404 page uses shared styling primitives (no major inline style blocks).
• Visual language matches the rest of docs.
• Primary recovery action (back to docs) is prominent and accessible.