fix: resolve failing tests across utility modules

[stooit]

Summary

Fixes all 16 failing tests (bun test: 44/16 → 60/0; tsc --noEmit clean). No test files or dependencies changed.

• calculator: divide throws Error("Division by zero") when b === 0.
• date-utils: formatRelative uses Math.round for day count (36h → "2 days ago"); removed the stale BUG comment.
• string-utils: wordCount splits on /\s+/; implemented truncate (word-boundary cut, ... counts toward maxLength).
• task-manager: implemented remove, update (only applies provided fields, false for unknown id), sortBy (priority high→low, status, createdAt oldest first; non-mutating).
• validator: isEmail accepts TLDs longer than 4 chars; isUrl no longer rejects URLs with ports (http/https allowlist kept).

Assumptions / notes

• sortBy("status") order assumed pending → in_progress → completed (untested; needed for type exhaustiveness).
• truncate with maxLength <= 3 hard-cuts without ellipsis (untested edge case; left as-is).
• sortBy("createdAt") ties within the same millisecond rely on stable sort/insertion order.