fix: consolidate sprint 1 QA findings

[LoadCG]

Summary

• Reject seed targets unless the database name is explicitly dedicated with a _dev or _test suffix.
• Build backend scripts in the seed validation CI job and document the local build prerequisite.
• Run browser E2E regression on relevant pull requests and main updates.
• Improve brand-link accessibility and restrict admin navigation/direct access to admin users.
• Allow an environment-based Vite API proxy target for isolated environments and fix an unused auth prop that blocked TypeScript builds.

Verification

• Backend: 318 tests passed with PostgreSQL integration tests enabled.
• Frontend: 195 tests passed; production build passed.
• Browser E2E: 22 tests passed in Chrome, including accessibility and mobile checks.
• Backend build, typecheck, seed validation, seed tests, dependency audit, and git diff --check passed.