Fix locale-aware timestamp formatting

[tenaxresearchbaekhui]

Summary

• switch Unix timestamp formatting to Intl.DateTimeFormat using the user's locale by default
• include short timezone names for date/time displays so timestamps are consistent across views
• reuse the shared time formatter in the market activity feed instead of calling toLocaleTimeString() directly
• add deterministic UTC coverage for the date/time helpers

Fixes #27

Validation

• node Intl.DateTimeFormat check for the UTC date and time fixture
• npx.cmd --yes -p typescript@5.7.3 tsc --noEmit --target ES2020 --lib ES2020,DOM --moduleResolution node frontend/src/utils/helpers.ts
• git diff --check

Note: I could not run the full Vitest suite from a clean clone because frontend/package.json is absent even though frontend/package-lock.json exists. A temporary npx Vitest attempt failed while loading vitest.config.ts because vitest/config is not resolvable from the repo without installed project dependencies.

[Muyideen-js]

@tenaxresearchbaekhui File conflict