fix: repair failing utility tests and implement missing functionality

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library. The full suite now passes: 60 pass, 0 fail. No test files were modified and no dependencies were added.

Changes by module

• src/calculator.ts — divide now throws on division by zero instead of returning Infinity.
• src/string-utils.ts — wordCount handles multiple consecutive spaces (trim + split on \s+); truncate implemented to respect word boundaries with the ellipsis counting toward maxLength and handling strings shorter than the ellipsis.
• src/task-manager.ts — implemented/completed remove (returns false for unknown id), update (title/priority, returns false for unknown id), and sortBy for priority (high → medium → low) and createdAt (oldest first).
• src/date-utils.ts — fixed formatRelative rounding so 36 hours reads "2 days ago"; the rounded hour value now drives both the < 24 branch guard and the label, so "24 hours ago" can no longer be emitted.
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts host:port URLs (e.g. http://localhost:3000).

Verification

• bun test → 60 pass, 0 fail, 70 expect() calls.
• Only the five src/*.ts files changed; no test files touched.

Review notes

Changes were reviewed by a review pass; follow-up polish applied: made the hours/days boundary consistent, fixed a lastSpace >= 0 boundary case in truncate, and removed stale BUG:/TODO: comments.

Assumptions

• Expected behaviours were inferred directly from the existing (unmodified) test cases.
• Error message text (e.g. "Division by zero") follows the assertions in the tests.