fix: resolve failing tests and implement missing utilities

[stooit]

Summary

• calculator.divide throws on division by zero
• date-utils.formatRelative rounds days (36h → "2 days ago"), future dates preserved
• string-utils: wordCount splits on any whitespace; truncate implemented (ellipsis counts toward maxLength, word-boundary cut)
• validator: isEmail accepts long TLDs (2–63 chars, 254-char cap); isUrl accepts URLs with ports (http/https allowlist kept)
• TaskManager: implemented remove, update, sortBy (priority/createdAt/status, returns copy)

Result: 60/60 tests pass, tsc --noEmit clean. No test files or dependencies changed.

Assumptions

• sortBy("status") is untested; ordered pending → in_progress → completed.
• update() treats undefined fields as "no change" (cannot clear description).
• isUrl relies on WHATWG URL parsing, so it normalises minor input quirks (e.g. surrounding whitespace).