fix: repair failing utility tests and implement missing functions

[stooit]

Summary

Fixes all 16 failing tests (now 60/60 pass) by repairing edge-case bugs and implementing missing functions across the utility library. No test files modified, no dependencies added — only src/ touched.

Changes

• calculator — divide now throws on a zero divisor instead of silently returning Infinity (also catches -0).
• validator — isEmail accepts long TLDs (e.g. .museum) with a 254-char length guard; isUrl no longer rejects URLs with a port (e.g. http://localhost:3000) while still allow-listing http(s).
• date-utils — day-diff rounds instead of flooring, fixing the off-by-one so 36h reads as "2 days ago".
• string-utils — wordCount collapses runs of whitespace (spaces/tabs/newlines); truncate implemented so the ellipsis counts toward maxLength, cuts at a word boundary, and never returns content-free output (including on leading whitespace).
• task-manager — implemented remove (returns removed/not-found bool), update (applies only present keys; required fields ignore explicit undefined, optional description can be cleared), and sortBy (priority high>medium>low, createdAt oldest-first; returns a sorted copy leaving internal order intact).

Verification

• bun test → 60 pass / 0 fail
• tsc --noEmit → clean
• Independent review pass: no blockers; the truncate leading-whitespace content-loss edge case flagged in review was fixed in a follow-up commit.

Assumptions

• sortBy("status") is not covered by any test; ordered in_progress > pending > completed (active work first). Documented as a judgement call — worth confirming intent.