fix: resolve failing tests and implement missing utilities

[stooit]

Summary

Fixes all 16 failing tests (now 60 pass / 0 fail; tsc --noEmit clean). No test files or dependencies changed.

• calculator: divide throws Error("Division by zero") when divisor is 0.
• date-utils: formatRelative day bucket uses Math.round(abs(hours)/24) so 36h → "2 days ago" (also corrects future-date rounding).
• string-utils: wordCount splits on /\s+/; truncate implemented (ellipsis counted in maxLength, cuts at word boundary, hard-slices when maxLength <= 3).
• task-manager: implemented remove, update (partial, returns false for unknown id), sortBy (priority high→medium→low, createdAt oldest-first, status).
• validator: isEmail accepts TLDs longer than 4 chars; isUrl accepts URLs with ports (http/https scheme allowlist retained).

Assumptions

• sortBy("status") order (in_progress → pending → completed) is not covered by tests; chosen as a reasonable default.
• truncate with maxLength <= 3 returns a plain slice without ellipsis (untested edge).