Fix locale-aware timestamp formatting

[nadeemkamel-arch]

Summary

• Add a shared formatTimestamp helper that formats Unix timestamps with the user locale/timezone by default and includes an explicit timezone label.
• Use the shared formatter in the market activity feed instead of a time-only toLocaleTimeString() call.
• Restore the missing frontend package.json from the existing lockfile/README scripts so frontend verification commands work, and fix two tiny existing CI blockers: Poppins weights and stale navbar brand expectations.

Verification

• npm run test
• npm run build
• npx tsc --noEmit
• git diff --check

Closes #27
/claim #27