fix: resolve all 16 failing tests across 5 source modules

[stooit]

Summary

• calculator.ts: Throw Error on division by zero instead of returning Infinity
• string-utils.ts: Fix wordCount to handle consecutive spaces; implement truncate with word-boundary-aware ellipsis
• task-manager.ts: Implement remove, update, and sortBy methods
• date-utils.ts: Use Math.round for day calculation in formatRelative (fixes 36h → "2 days ago")
• validator.ts: Allow long TLDs (e.g. .museum) in isEmail; support ports and localhost in isUrl

Test Results

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

Assumptions

• truncate uses word-boundary truncation: walks back from maxLength - 3 to find a space, appends ...
• sortBy("priority") orders high > medium > low
• sortBy("createdAt") orders oldest first
• Day rounding in formatRelative uses Math.round (36h = 1.5 days → 2 days)