fix: make all failing tests pass across utility library

[stooit]

Summary

Fixes all 16 previously failing tests (60/60 now green) across the TypeScript utility library. Only src/ files changed — no test files modified, no new dependencies.

Changes

• src/calculator.ts — divide now throws on a zero divisor instead of returning Infinity (explicit b === 0 guard, also catches -0).
• src/string-utils.ts — implemented truncate (ellipsis counted within maxLength, word-boundary aware); fixed wordCount to split on /\s+/ so runs of whitespace/tabs/newlines don't produce phantom empty words.
• src/task-manager.ts — implemented stubbed remove, update (partial updates only touch provided keys), and sortBy (priority/status rank maps + createdAt timestamp; sorts a copy).
• src/date-utils.ts — formatRelative day calc uses Math.round instead of Math.floor, so 36h reads "2 days ago".
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum) while still rejecting empty labels; isUrl no longer rejects URLs with ports (e.g. http://localhost:3000), protocol still allowlisted to http/https.

Verification

• bun test → 60 pass / 0 fail (stable across repeated runs)
• tsc --noEmit clean
• Independent review pass: no over-fitting to individual assertions; each fix generalises past its test. Verdict: safe to commit.

Assumptions & notes

• divide throwing is an API-visible behaviour change vs. the previous Infinity return, sanctioned by the in-code BUG: comment.
• Two edge cases outside current test coverage were noted for possible follow-up (non-blocking): negative maxLength in truncate, and clearing an optional description field via update.
• isUrl/isEmail are syntactic validators only; if isUrl ever guards a redirect/fetch target, protocol allowlisting alone won't prevent SSRF to internal addresses.