fix: resolve all failing tests in utility library

[stooit]

Summary

Fixes all 16 failing tests across the 5 utility modules. The full suite now passes (60 pass / 0 fail). No test files were modified and no dependencies were added.

Changes

File	Fix
src/calculator.ts	divide now throws Error("Division by zero") instead of returning Infinity
src/string-utils.ts	wordCount splits on /\s+/ so consecutive spaces count as one separator; truncate implemented with word-boundary truncation where the "..." ellipsis counts toward maxLength
src/task-manager.ts	Implemented remove (returns false for unknown id), update (returns false for unknown id, applies provided fields), and sortBy (priority: high→medium→low; createdAt: oldest first)
src/date-utils.ts	formatRelative uses Math.round for the days calculation, fixing the off-by-one (36h → "2 days ago")
src/validator.ts	isEmail accepts long TLDs ({2,}); isUrl accepts URLs with ports

Verification

• bun test → 60 pass, 0 fail, 70 expect() calls
• Independent review confirmed all fixes correct with no blockers

Assumptions

• Scope limited to what the tests require, per task constraints. A review flagged two pre-existing, untested edge cases (formatRelative rounding at the 12–24h boundary; truncate behaviour when maxLength < 3) — left unchanged to avoid altering behaviour the test suite does not exercise.