fix: resolve all 16 failing tests across 5 utility modules

[stooit]

Summary

• calculator.ts: Added divide-by-zero guard that throws an Error instead of returning Infinity
• string-utils.ts: Fixed wordCount to handle consecutive whitespace; implemented truncate with word-boundary logic
• task-manager.ts: Implemented missing remove, update, and sortBy methods
• date-utils.ts: Changed formatRelative from Math.floor to Math.round for day calculation
• validator.ts: Updated isEmail regex to allow TLDs longer than 4 chars; updated isUrl regex to support port numbers

Test Results

All 60 tests pass (previously 44 pass / 16 fail).

Assumptions

• truncate uses word-boundary truncation where "..." counts toward maxLength
• sortBy("priority") orders high > medium > low
• sortBy("createdAt") orders oldest first
• No test files were modified
• No new dependencies were added