cleanup: migrate inline Math.floor(Date.now()/1000) to nowUnix() helper

[ignromanov]

Context

PR #115 introduced src/shared/lib/date-time/index.ts with nowUnix() and daysFromNowUnix(n) helpers to centralize Unix timestamp creation. However, the migration was incomplete — several production files still use the raw Math.floor(Date.now() / 1000) pattern.

Locations to migrate

Production code (must fix):

• src/features/rpc-proxy/lib/mock.ts — lines 117, 230, 304, 323
• src/widgets/landing/constants/demo-invoices.ts — line 31

Test utilities (optional):

• src/shared/lib/test-utils/invoice-generator.ts — line 140 (test utils intentionally capturing raw Date.now() is acceptable; migrate only if it improves consistency)

Proposal

Replace all production occurrences with nowUnix() imported from @/shared/lib/date-time. No behavior change — purely mechanical substitution.

Acceptance Criteria

• Math.floor(Date.now() / 1000) replaced with nowUnix() in mock.ts (4 occurrences) and demo-invoices.ts (1 occurrence)
• test-utils/invoice-generator.ts — migrated or left with a comment explaining the intentional raw usage
• pnpm type-check:build && pnpm lint pass
• All tests pass

Links

• PR feat(payment): typed wagmi error handling + onReplaced support #115 — introduced nowUnix() helper
• src/shared/lib/date-time/index.ts — helper implementation