fix: repair 16 failing utility-library tests

[stooit]

Summary

Fixes all 16 failing tests across 5 modules so the full suite passes (60 pass / 0 fail). No test files were modified and no dependencies were added — only the behaviour the tests require was changed.

Changes

• src/calculator.ts — divide now guards against a zero divisor (throws). The b === 0 check also catches -0, which previously produced -Infinity.
• src/string-utils.ts
  • wordCount now trims and splits on /\s+/, correctly counting words separated by consecutive spaces (and tabs/newlines).
  • truncate implemented: returns the string unchanged when str.length <= maxLength; otherwise truncates at a word boundary with "..." counting toward maxLength.
• src/task-manager.ts — implemented remove (delete by id), update (partial field update, returns false if not found), and sortBy (returns a sorted copy, leaving internal insertion order intact).
• src/date-utils.ts — fixed off-by-one in relative-day formatting by rounding the (absolute) day difference instead of flooring, so 36h correctly reads "2 days ago" and future dates stay symmetric.
• src/validator.ts — isEmail broadened to accept valid multi-label domains / longer TLDs while still rejecting malformed labels; isUrl now accepts URLs with explicit ports (e.g. http://localhost:3000) while retaining the protocol allowlist.

Verification

• bun test → 60 pass / 0 fail
• bunx tsc --noEmit → clean
• Independent review pass: no blocking correctness issues; remaining notes concern untested edge cases and pre-existing main behaviour, left unchanged per the task scope ("fix only what the tests require").

Assumptions

• Scope limited to what the tests require; untested edge cases (e.g. negative maxLength, clearing an optional description via update) were intentionally left as-is to avoid changing unverified behaviour.