fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests across the utility library. The full suite now passes: 60 pass / 0 fail.

Changes

• src/string-utils.ts — implemented wordCount (collapses consecutive whitespace via \s+ split after trim) and truncate (returns unchanged if within limit, reserves 3 chars so the ellipsis counts toward maxLength, truncates on the last word boundary, and handles strings shorter than the ellipsis).
• src/task-manager.ts — implemented remove (returns true/false), update (title/priority, returns false for unknown id), and sortBy for priority (high → medium → low) and createdAt (oldest first).
• src/validator.ts — fixed isEmail to accept long TLDs (e.g. .museum) and isUrl to accept URLs with a port.
• src/calculator.ts — divide now throws on division by zero.
• src/date-utils.ts — fixed off-by-one in relative-day formatting so 36h reads as expected.

Testing

bun test → 60 pass, 0 fail, 70 expect() calls.

Notes / assumptions

• No test files were modified and no dependencies were added, per task constraints.
• sortBy("priority") treats high as most-important-first; sortBy("createdAt") returns oldest first — both per the test contracts.