fix: resolve failing tests and implement missing utilities

[stooit]

Summary

Fixes all 16 failing tests (now 60/60 pass, tsc --noEmit clean). No test files modified, no dependencies added.

• calculator: divide throws Error("Division by zero") when b === 0.
• date-utils: formatRelative rounds days (Math.round(Math.abs(diffHours)/24)) instead of flooring, matching the test's assertion at 36h; symmetric for past/future.
• string-utils: wordCount splits on /\s+/ after trim; truncate implemented (word-boundary cut, ... counts toward maxLength, hard-slice when maxLength <= 3, clamped for negatives).
• validator: isEmail TLD bound {2,4} → {2,} (minimal change); isUrl no longer rejects URLs with a port.
• task-manager: implemented remove, update, sortBy (priority high→low, status, createdAt oldest-first; returns a copy).

Assumptions

• Day rounding behaviour follows the test's actual assertion (the test title wording "1 day ago" differs from its expected value).
• update allows explicitly clearing description by passing undefined; title/priority ignore undefined.
• truncate with negative maxLength returns "" (defensive, untested).
• Review feedback applied: kept isEmail local-part permissive (original [^\s@]+) to avoid rejecting valid addresses like o'brien@….