fix: repair 16 failing tests across utility library

[stooit]

Summary

Fixes all 16 previously failing tests across 5 source files. 60/60 tests pass, tsc --noEmit clean. No test files modified, no dependencies added.

Fixes

• calculator.ts — divide now throws on division by zero instead of returning Infinity.
• string-utils.ts — wordCount collapses consecutive-whitespace runs (/\s+/); implemented truncate to cut at a word boundary with "..." counting toward maxLength, returning the string unchanged when within limit.
• task-manager.ts — implemented remove (true/false on found), update (title/description/priority, true/false on found), and sortBy (priority high>medium>low, createdAt oldest-first, plus status ordering).
• date-utils.ts — formatRelative off-by-one fixed by rounding hours-to-days (Math.round), so 36h → "2 days ago"; symmetry preserved for future dates.
• validator.ts — isEmail accepts long TLDs (e.g. .museum) while still rejecting a@b..com; isUrl accepts URLs with ports (e.g. http://localhost:3000).

Verification

• bun test: 60 pass, 0 fail
• tsc --noEmit: clean
• Reviewed via review subagent — verdict COMMENT, no blockers.

Assumptions

• sortBy("status") order (in_progress > pending > completed) and the numeric id tiebreak for same-millisecond createdAt ties are not exercised by tests; chosen as reasonable defaults.
• Review raised non-blocking suggestions (truncate whitespace-boundary detection, update clearing optional fields, SSRF/credential-URL considerations for isUrl) that go beyond what the tests require; left out per the "fix only what the tests require" constraint.