fix(leaderboard): normalize timezone display in timestamp helpers

[praveenchand-2005]

Summary

Fixes the inconsistent timezone display on the leaderboard by repairing the timestamp helpers.

The original helpers.ts contained merge-conflict artifacts:

• Duplicate toTimestampMs - the first copy returned Date.now() for invalid input (so bad timestamps rendered as "now"), the second was the correct normalizer.
• Duplicate formatDate, formatTime, timeAgo, formatEventTime definitions.
• formatDate used a hardcoded inline threshold (4_102_444_800) instead of the shared toTimestampMs normalizer, so seconds/millisecond handling could diverge from every other call site.

Changes

• Removed all duplicate definitions and dead code.
• formatDate / formatTime now route through the single shared toTimestampMs, so a Unix-seconds timestamp and the same instant in milliseconds render identically in the viewer's local timezone with a timezone label.
• formatEventTime now uses Intl.DateTimeFormat consistently with the rest of the file (previously mixed toLocaleString).
• Added tests for formatEventTime, including invalid-input handling.

Test evidence

vitest run - 15 files / 217 tests passing:

• helpers.test.ts: 66 tests passing (formatDate/formatTime timezone-aware, seconds+ms normalization, invalid input -> em dash, formatEventTime).
• Full suite: 217 passed.

Verified with tsc --noEmit - no errors.

[praveenchand-2005]

/claim #80

[praveenchand-2005]

Test & Check Evidence (verified locally)

Vitest (full suite):

Test Files  15 passed (15)
Tests       217 passed (217)
Duration    57.90s

• helpers.test.ts: 66 tests passing, covering timezone-aware formatDate/formatTime, seconds+ms normalization, invalid input -> em dash, and formatEventTime (incl. invalid-input handling).
  TypeScript: tsc --noEmit - no errors.

PR status: Changes can be cleanly merged. Repo has no CI workflow configured (Checks 0), so verification is provided via the local suite above.

Summary of fix: Removed merge-conflict duplicate definitions in helpers.ts (the first toTimestampMs returned Date.now() for invalid input; formatDate used an inline threshold instead of the shared normalizer). Now all timestamps render consistently in the viewer's locale/timezone.