fix: repair failing tests and implement missing utility functions

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library and implements the missing functionality. All 60 tests now pass; no test files were modified and no dependencies were added.

Changes (source only)

• src/calculator.ts — divide() now throws on division by zero instead of silently returning Infinity.
• src/date-utils.ts — formatRelative() rounds days to the nearest value (36h → "2 days ago") and is symmetric for future dates by taking Math.abs before rounding.
• src/validator.ts — isEmail() accepts subdomains and long TLDs (e.g. .museum); isUrl() accepts URLs with a port (e.g. http://localhost:3000). Protocol allow-listing (http/https) retained.
• src/string-utils.ts — wordCount() collapses consecutive whitespace; implemented truncate() with word-boundary truncation, ellipsis counting toward maxLength, whitespace-aware boundaries, and a non-positive maxLength guard.
• src/task-manager.ts — implemented remove(), update(), and sortBy() (priority high>medium>low, status workflow order, createdAt oldest-first).

Verification

bun test  →  60 pass / 0 fail
tsc --noEmit  →  clean

Review notes / assumptions

A review pass caught two extra defects in previously-untested branches, both now fixed: formatRelative future-date asymmetry, and truncate splitting mid-word on tabs/newlines / over-length output for negative maxLength.

Assumptions:

• sortBy("createdAt") relies on Array.prototype.sort stability (ES2019+) for same-millisecond ties.
• update({ description: undefined }) does not clear the field (partial-update semantics).
• isUrl is a protocol/format check, not an SSRF control — unchanged from prior behaviour.