fix: repair failing tests across calculator, string, date, task, and validation utils

[stooit]

Summary

Fixes all 16 previously failing tests (now 60 pass / 0 fail) across five utility modules. No test files were modified and no dependencies were added — only the source functions the tests exercise were changed.

Changes

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0, instead of silently returning Infinity.
• src/string-utils.ts — wordCount splits on /\s+/ (collapses consecutive whitespace, returns 0 for whitespace-only); truncate implemented with word-boundary back-off and ellipsis counting toward maxLength, never exceeding it.
• src/task-manager.ts — implemented remove (hit/miss), update (safe partial updates), and sortBy (priority/status rank maps, oldest-first on createdAt).
• src/date-utils.ts — formatRelative rounds the day count, fixing the off-by-one (36h → "2 days"); past/future branches symmetric.
• src/validator.ts — isEmail accepts TLDs > 4 chars; isUrl accepts host:port URLs while keeping the http/https allowlist.

Verification

• bun test → 60 pass, 0 fail
• tsc --noEmit → clean
• Changes independently reviewed; a date-utils rounding regression and a truncate word-boundary over-fit found in review were corrected before commit.

Assumptions / notes

• isEmail remains a permissive shape check, not RFC 5322 validation.
• test/ left untouched per the brief; a couple of untested edge paths noted for follow-up, out of scope here.