fix: resolve all 16 failing tests across utility library

[stooit]

Summary

• calculator.ts: Throw error on division by zero instead of returning Infinity
• date-utils.ts: Use Math.round for day calculation in formatRelative (36h → "2 days ago")
• 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
• validator.ts: Allow longer TLDs (e.g. .museum) in isEmail; support port numbers in isUrl

Test Results

All 60 tests pass, 0 failures.

Assumptions

• formatRelative day calculation uses Math.round (test expects 36h = "2 days ago")
• truncate truncates at word boundaries with "..." suffix within maxLength
• sortBy("priority") orders high > medium > low
• No test files were modified; no new dependencies added