fix: resolve all failing tests across utility modules

[stooit]

Summary

Fixes all 16 failing tests across the utility library. All 60 tests now pass (0 failures), with no previously-passing tests broken and no new dependencies added.

Changes

• src/calculator.ts — divide now throws Error("Division by zero") instead of returning Infinity.
• src/date-utils.ts — formatRelative now uses Math.round (was Math.floor) for the hours-to-days conversion, fixing the off-by-one (36h → "2 days ago").
• src/string-utils.ts — implemented truncate (ellipsis counts toward maxLength, word-boundary truncation, returns unchanged when within limit, handles strings shorter than the ellipsis) and fixed wordCount to collapse consecutive whitespace.
• src/task-manager.ts — implemented/completed remove, update, and sortBy (by priority high→medium→low, and by createdAt oldest-first).
• 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) via the URL constructor.

Testing

bun test → 60 pass, 0 fail.

Notes / assumptions

• Test files were not modified.
• A reviewer noted a pre-existing cosmetic edge in formatRelative (a date ~23.5h ago can render as "24 hours ago"); this predates the change, is not covered by any test, and was left out of scope per the "fix only what the tests require" constraint.