fix: repair all failing utility-library tests

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library. The full suite now passes (60 pass / 0 fail).

Changes

• src/calculator.ts — divide now throws Error("Division by zero") on a zero divisor (b === 0) instead of returning Infinity.
• src/validator.ts — isEmail TLD pattern widened from {2,4} to {2,} (accepts long TLDs); isUrl reimplemented with the URL constructor + protocol check so ports, query strings and fragments validate correctly.
• src/string-utils.ts — wordCount splits on /\s+/ after trimming (collapses consecutive spaces); truncate cuts at a word boundary with the "..." ellipsis counting toward maxLength.
• src/task-manager.ts — remove/update return false for unknown IDs; sortBy orders by priority (high → medium → low) and by createdAt (oldest first).
• src/date-utils.ts — formatRelative off-by-one fixed (36h now reports "2 days ago").

Verification

• bun test → 60 pass, 0 fail.
• Independent correctness review confirmed all fixes are genuine implementations, not test-specific over-fits. Two non-blocking nits noted (formatRelative rounding near the ~23h30m boundary; truncate returning input unchanged when maxLength <= 3) — both pre-existing, untested, out of scope.

Assumptions

• No test files modified; no dependencies added — only src/ changed, per task constraints.