fix: resolve failing tests across utility modules

[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 b === 0.
• date-utils: formatRelative rounds days (Math.round(|h|/24)) so 36h reads as 2 days, matching the test.
• string-utils: wordCount splits on /\s+/ after trim; truncate implemented (ellipsis counted in maxLength, cuts at word boundary, handles maxLength < ellipsis).
• validator: isEmail supports subdomains and long TLDs (2–63 chars), keeps permissive (incl. non-ASCII) local part, 254-char cap; isUrl no longer rejects URLs with a port (scheme still limited to http/https).
• task-manager: implemented remove, update, sortBy (priority via explicit rank map high>medium>low; createdAt oldest first; returns a copy).

Assumptions / notes

• Test expectations define behaviour where ambiguous (e.g. rounding for day boundary).
• Review follow-ups not addressed (out of scope, no tests): update() can't clear description via undefined; formatRelative can emit "24 hours ago" for 23.5–23.9h; isUrl does no hostname validation (pre-existing).