fix: repair failing tests across utility library

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library. Test suite now passes 60/60 (was 44/60), with no changes to test files and no new dependencies.

Changes

• src/calculator.ts — divide now throws Error("Division by zero") on a zero divisor instead of returning Infinity.
• src/string-utils.ts — wordCount handles consecutive/leading/trailing whitespace via trim().split(/\s+/); truncate implemented.
• src/task-manager.ts — implemented the missing/incomplete TaskManager methods (remove / update / sortBy).
• src/date-utils.ts — corrected formatRelative off-by-one rounding so relative durations bucket correctly.
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts explicit ports (e.g. http://localhost:3000).
• Refreshed stale BUG: JSDoc comments that described the now-fixed behaviour.

Verification

• bun test → 60 pass / 0 fail (70 assertions).
• Independent review pass (verdict: ship). No test files modified; no dependencies added; changes kept minimal and consistent with existing style.

Assumptions

• Division by zero should raise an error (matches the failing test expectation) rather than return a sentinel value.
• isUrl/isEmail relaxations are intentional per the tests; the review noted these widen accepted input but flagged no correctness regression within tested scope.