Post-V1: reduce existing lint warnings

[NexaStack-Software]

The v1.0.0 release gate passes with lint warnings but no lint errors. This issue tracks reducing those warnings after the stable release.\n\nScope:\n- Start with unused variables and low-risk type assertion warnings.\n- Avoid behavior changes and broad refactors.\n- Keep fixes split into reviewable commits.\n- Verify lint, typecheck, and focused tests after each cleanup batch.

[NexaStack-Software]

Implemented in 8b59146. Reduced lint warnings in the post-release lint surface from 61 to 29 across @nexasign/lib, @nexasign/ui, and @nexasign/trpc by removing unused symbols, using existing className props, and replacing several unsafe casts with type guards/typed mappings. Verification: targeted lint passes with 0 errors and 29 remaining warnings; full V1 release gate passed through prisma generate, typecheck, lint, build, migrate deploy, and 18/18 release E2E tests. Final strict demo drift after push: 14 PASS, 0 WARN, 0 FAIL.