Enforce docs/DESIGN.md as single source of truth across frontend

[georgi]

Removed duplicate DESIGN.md files from web/ and marketing/ directories. Updated references in web/PRODUCT.md and marketing/archive/README.md to point to the canonical docs/DESIGN.md file to ensure consistency and single-source truth for design guidelines across the monorepo.

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PR created automatically by Jules for task 5010075453890156743 started by @georgi

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