fix: repair failing tests across utility library

[stooit]

Summary

Fixes all 16 previously-failing tests across 5 source files. All 60 tests now pass; tsc --noEmit is clean. Only source files were modified — no test files changed, no dependencies added.

Changes

• src/calculator.ts — divide now throws on division by zero instead of returning Infinity (also catches -0).
• src/string-utils.ts — wordCount splits on /\s+/ (collapses consecutive whitespace); implemented truncate to cut at a word boundary with "..." counting toward maxLength, guarding the maxLength <= 3 degenerate case.
• src/task-manager.ts — implemented remove, update (per-field, avoids clobbering with undefined), and sortBy (priority high>medium>low, createdAt oldest-first; sorts a copy so the internal Map isn't mutated on read).
• src/date-utils.ts — formatRelative day calc uses Math.round(abs(hours)/24) instead of Math.floor, fixing the 36h→"2 days ago" off-by-one and the future-date branch.
• src/validator.ts — isEmail accepts long TLDs ({2,}); isUrl accepts explicit ports (dropped the erroneous port === "" check) while keeping the http/https allowlist.

Assumptions & notes

• Test files were treated as the source of truth for expected behaviour; changes are minimal to satisfy them.
• isEmail remains a permissive format check (not full validation) and isUrl accepts any host including internal ones (localhost, link-local). Both match the tests but are not security controls — if either gates a server-side fetch or ingress path, add SSRF/host-allowlist defences and confirmation-link email verification at the call site.

Testing

bun test → 60 pass / 0 fail.