fix: resolve all failing tests across utility library

[stooit]

Summary

• calculator.ts: Throw error on division by zero instead of returning Infinity
• string-utils.ts: Fix wordCount to handle multiple consecutive spaces; implement truncate with word-boundary logic
• task-manager.ts: Implement missing remove, update, and sortBy methods
• date-utils.ts: Use Math.round instead of Math.floor for relative day calculation
• validator.ts: Expand email TLD regex to support longer TLDs (e.g. .museum); fix URL regex to handle ports

Testing

All 60 tests pass (bun test). No test files were modified.

Assumptions

• truncate uses "..." (3 chars) as the ellipsis and counts it toward maxLength
• sortBy("priority") sorts high > medium > low (descending severity)
• Division by zero throws a generic Error with message "Division by zero"