chore: add spec for dashboard CSS layer restructuring

[augustina-jpg]

closes #686
closes #687
closes #688
closes#695

• Create requirements document (Req 1-12) following ITCSS methodology
• Define base, layout, components, and utilities layers
• Establish zero-change visual regression testing
• Document acceptance criteria for all browser and viewport combinations
• Reference tests/dashboard_asset_tests.rs for pass-through validation

Part of #627 (59/100)

What and why

Closes #

How it was verified

• cargo test passes
• cargo fmt --check and cargo clippy --all-targets -- -D warnings pass
• New behaviour is covered by a test that fails without this change

Risk

• Adds or changes a database migration (forward-only — see DEPLOYMENT.md)
• Changes a public API response shape or error code
• Touches payment verification, settlement, or amount handling
• Touches authentication, the SSRF guard, or webhook signing
• Adds or updates a dependency (cargo deny check all passes)
• Requires a config/env change to deploy (documented in README + .env.example)

Notes for the reviewer

[drips-wave]

@augustina-jpg Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits