fix: repair failing tests across utility library

[stooit]

Summary

Fixes all 16 previously-failing tests across the TypeScript utility library. All 60 tests now pass; no previously-passing tests were broken. No test files modified, no new dependencies added.

Changes

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0, instead of returning Infinity.
• src/string-utils.ts — wordCount uses trim().split(/\s+/) so consecutive spaces don't yield empty tokens; truncate ellipsis accounting corrected.
• src/task-manager.ts — completed missing/incomplete TaskManager methods (incl. completeTask setting completedAt).
• src/date-utils.ts — formatRelative off-by-one fixed (Math.round instead of Math.floor), so e.g. 36 hours ago → "2 days ago".
• src/validator.ts — isEmail now accepts long TLDs (e.g. user@example.museum); isUrl now accepts URLs with ports (e.g. http://localhost:3000).

Verification

• bun test → 60 pass, 0 fail (70 expect() calls across 5 files).
• Independent review pass: approved. One non-blocking nit noted — truncate with maxLength ≤ 2 hard-slices without an ellipsis; not covered by tests and behaviour is unspecified, so left as-is per the "fix only what tests require" constraint.

Assumptions

• The email/URL regexes are intentionally permissive to satisfy the test contract; no stricter RFC validation was introduced since tests don't require it.
• completeTask sets completedAt to wall-clock new Date() rather than an injected timestamp.