fix: make all tests pass — implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests across the utility library (60/60 pass, previously 44/60). Only source files under src/ were changed — no test files edited, no dependencies added, per the task constraints.

Changes

• src/calculator.ts — divide now throws on division by zero instead of silently returning Infinity.
• src/string-utils.ts — wordCount splits on /\s+/ (handles consecutive spaces, tabs, newlines); implemented truncate with word-boundary cutting and an ellipsis budget so output never exceeds maxLength.
• src/task-manager.ts — implemented the missing remove, update, and sortBy methods. sortBy uses typed rank maps (high > medium > low) and ascending createdAt (oldest first).
• src/date-utils.ts — fixed off-by-one in formatRelative day bucketing (Math.round instead of Math.floor); Math.abs moved before rounding so past/future dates stay symmetric.
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts port-bearing URLs (e.g. http://localhost:3000) while keeping the protocol allowlist (rejects ftp:, javascript:, data:).

Verification

• bun test → 60 pass / 0 fail
• tsc --noEmit (strict) → clean

Notes & assumptions

• Fixes were kept minimal and scoped to what the tests require, as instructed. A review pass flagged non-blocking hardening opportunities (negative/NaN maxLength guards in truncate, a runtime default in sortBy, ability to clear optional fields in update) — all outside the test requirements and left as-is to avoid scope creep.
• Security note for downstream callers: isUrl is now port-agnostic (http://localhost:3000, http://[::1]:8080 pass), so it must not be relied on as an SSRF/open-redirect allowlist. isEmail remains a structural check, not RFC 5322 validation or proof of deliverability.