fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Makes all failing tests pass in the TypeScript utility library. The suite went from 44/60 → 60/60 passing, with no test files modified and no new dependencies added.

Changes by file

• src/calculator.ts — divide() now throws "Division by zero" instead of returning Infinity.
• src/string-utils.ts — wordCount() collapses consecutive whitespace (trim().split(/\s+/)); truncate() implemented with word-boundary truncation, ellipsis counted toward maxLength, and explicit handling for tiny/zero maxLength.
• src/task-manager.ts — implemented remove(), update(), and sortBy() (priority: high→medium→low; createdAt: oldest first). update() uses explicit per-key assignment so callers can't silently overwrite status/completedAt.
• src/date-utils.ts — formatRelative() off-by-one fixed (rounds instead of floors, so 36h reads as "2 days ago").
• src/validator.ts — isEmail() accepts long TLDs; isUrl() accepts URLs with a port.

Verification

• bun test → 60 pass, 0 fail (70 assertions).
• No files under test/ changed.

Assumptions

• Followed the behaviour encoded in the existing tests as the spec (e.g. division-by-zero throws; relative-time rounds).
• Applied a few low-risk hardening tweaks surfaced in review (explicit update guards, degenerate truncate input handling, stale BUG: comment cleanup) — all kept the suite green.