test: fix timezone issues in EventDisplay tests

- Set explicit UTC timezone globally in jest.config.ts and setupTests.ts
- Mock Intl.DateTimeFormat to use UTC by default while preserving prototype
- Replace regex patterns with exact values for declarative assertions
- Remove complex date formatting computations in tests
- Use fixed dates (2025-01-15T10:00:00.000Z) for consistent test results
- Fix EventDisplay.test.tsx to use exact time values instead of regex

jest.config.ts

@@ -1,5 +1,8 @@
 import type { Config } from "jest";
 
+// Set timezone to UTC for consistent test results across all environments
+process.env.TZ = "UTC";
+
 const config: Config = {
   collectCoverage: true,
   coverageDirectory: "coverage",