fix: repair all failing utility-library tests

[stooit]

Summary

Fixes all 16 failing tests across the TypeScript utility library so the full suite passes (60/60), without modifying any tests or adding dependencies.

Changes

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0 instead of returning Infinity.
• src/date-utils.ts — formatRelative uses Math.round instead of Math.floor for the hours→days conversion, fixing the off-by-one (e.g. 36h → "2 days ago").
• src/string-utils.ts — implemented the missing utility function(s) required by the tests.
• src/task-manager.ts — completed the missing/incomplete TaskManager methods.
• src/validator.ts — broadened validation to handle edge cases the tests cover (e.g. isUrl with a port).

Verification

bun test → 60 pass, 0 fail.

Assumptions & scope notes

• Changes kept minimal and strictly driven by what the tests assert. No test files modified, no dependencies added.
• A review pass flagged a few latent, untested edge cases left out of scope (e.g. truncate with maxLength ≤ 3, sortBy tiebreaking on equal timestamps, empty titles in TaskManager). None affect the suite — noted for future follow-up.