fix: repair all failing utility functions

[stooit]

Summary

Fixes all 16 failing tests across the utility library. All 60 tests now pass (0 fail).

Changes

• calculator.ts — divide now throws Error("Division by zero") instead of returning Infinity.
• date-utils.ts — formatRelative uses Math.round (was Math.floor) so 36 hours ago rounds to "2 days ago".
• string-utils.ts — implemented truncate (returns unchanged within limit, truncates at word boundary, ellipsis counts toward maxLength, handles strings shorter than the ellipsis); fixed wordCount to collapse consecutive spaces.
• task-manager.ts — implemented/completed remove (false for unknown id), update (title & priority, false for unknown id), and sortBy (priority high>medium>low, createdAt oldest first).
• validator.ts — fixed isEmail for long TLDs and isUrl for URLs with a port (e.g. http://localhost:3000).

Verification

bun test -> 60 pass, 0 fail.

Assumptions & notes

• Scope kept strictly to what the tests require, per task constraints. No test files modified; no dependencies added.
• A review flagged that the isEmail/isUrl patterns remain permissive for some structurally-odd inputs not covered by tests. Tightening them was deliberately left out to avoid regressing passing tests and to respect the "fix only what the tests require" constraint.