fix: repair failing tests across utility library

[stooit]

Summary

Fixes all 16 failing tests across the TypeScript utility library. All 60 tests now pass with no dependencies added and no test files modified.

Changes

• src/calculator.ts — divide now throws "Division by zero" instead of returning Infinity.
• src/date-utils.ts — formatRelative uses Math.round for day count, fixing the off-by-one (36h → "2 days ago").
• src/string-utils.ts — implemented truncate (word-boundary aware, ellipsis counts toward maxLength, handles strings shorter than ellipsis) and slugify; fixed wordCount to collapse consecutive whitespace.
• src/validator.ts — isEmail now handles subdomains/long TLDs; isUrl accepts URLs with ports.
• src/task-manager.ts — implemented remove, update, and sortBy (priority high>medium>low; createdAt oldest first).

Verification

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

Assumptions / notes

• Per the "fix only what tests require" constraint, a pre-existing latent edge case in formatRelative (23h59m renders as "24 hours ago" rather than "1 day ago") was left untouched — not covered by any test, not introduced here. Worth a future follow-up.