fix: repair failing tests across utility library

[stooit]

Summary

Fixes all 16 failing tests across the library. All 60 tests now pass (0 fail). No test files modified, no dependencies added.

Changes

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0 (previously returned Infinity).
• src/date-utils.ts — formatRelative uses Math.round instead of Math.floor for the hours→days conversion, fixing the off-by-one (36h now → "2 days ago").
• src/string-utils.ts — implemented wordCount collapsing of multiple consecutive spaces, and the truncate word-boundary logic (ellipsis counts toward maxLength).
• src/task-manager.ts — implemented the missing/incomplete remove, update, and sortBy (priority and createdAt) methods.
• src/validator.ts — isEmail now accepts long TLDs (e.g. user@example.museum); isUrl now accepts URLs with a port (e.g. http://localhost:3000).

Verification

• bun test → 60 pass / 0 fail.
• Independent review pass confirmed fixes are correct, idiomatic, and introduce no regressions.

Assumptions

• Followed "fix only what the tests require": pre-existing edge cases outside the tested range (e.g. truncate with maxLength <= ellipsis.length) were left untouched to avoid altering the established contract.