fix: resolve all failing tests across 5 utility modules

[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 remove, update, and sortBy methods
• date-utils.ts: Use Math.round for day calculation in formatRelative (fixes 36h showing as "1 day ago" instead of "2 days ago")
• validator.ts: Allow long TLDs (e.g. .museum) in isEmail; allow ports on hostnames without dots (e.g. localhost:3000) in isUrl

Test Results

All 60 tests pass across 5 test files. No previously passing tests were broken.

Assumptions

• truncate truncates at word boundaries with "..." appended, where the total length including ellipsis does not exceed maxLength
• sortBy("priority") sorts high > medium > low
• sortBy("createdAt") sorts oldest first
• Division by zero throws a generic Error with message "Division by zero"