fix: repair failing tests and implement missing utility functions

[stooit]

Summary

Fixes all 16 failing tests across the utility library. Test suite now: 60 pass / 0 fail. No test files were modified and no dependencies were added.

Changes

• calculator.ts — divide now throws on division by zero instead of returning Infinity.
• string-utils.ts — fixed wordCount to collapse consecutive whitespace; implemented truncate with ellipsis + word-boundary handling.
• task-manager.ts — completed the missing/incomplete TaskManager methods.
• date-utils.ts — fixed the off-by-one in formatRelative's day rounding.
• validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts ports (e.g. http://localhost:3000).

Review hardening

A review pass caught three edge-case regressions in the initial fix (all near tested boundaries, none covered by existing tests) which were corrected:

• date-utils hours-branch guard no longer rounds, removing a 23.5–24h dead zone.
• truncate returns an ellipsis at maxLength=3 (boundary budget of 0).
• truncate no longer discards a word boundary at index 0.

Assumptions

• Tests are the source of truth for expected behaviour; implementations were aligned to them.
• Error messages and relative-time wording match the assertions in the test files.

Testing

bun test → 60 pass, 0 fail, 70 expect() calls.