chore(ui): remove retired Design Lab

[hmjn023]

Summary

• Remove the mock-only Design Lab route and screen now that V2 is the executable UI reference.
• Remove the Design Lab navigation prop/links and the root route special case.
• Update DESIGN.md, REPORT.md, and the indexion dependency graphs.
• Leave V1 screens, APIs, and data behavior unchanged.

Validation

• Server production build passed.
• Root typecheck passed.
• Design lint and targeted Biome checks passed.
• UI unit tests: 98 passed.
• Server unit tests: 198 passed.
• Server integration tests: 66 passed.
• Production responsive app navigation E2E: 4 passed.
• V2 routes responsive E2E: 14 passed, 14 failed in existing V2 sidebar/URL/short-viewport assertions; no V2 implementation files are changed by this PR.

[coderabbitai]

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