fix: resolve all 16 failing tests across utility library

[stooit]

Summary

• calculator.ts: Throw Error on divide-by-zero instead of returning Infinity
• string-utils.ts: Fix wordCount to handle multiple consecutive spaces; implement truncate with word-boundary logic
• date-utils.ts: Use Math.round instead of Math.floor for relative day calculation (36h → "2 days ago")
• validator.ts: Allow TLDs of 2+ characters in isEmail; support port numbers in isUrl
• task-manager.ts: Implement missing remove, update, and sortBy methods

Test Results

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

Assumptions

• formatRelative uses Math.round for day boundaries (matches test expectations)
• truncate truncates at word boundary when possible, hard-truncates at maxLength - 3 otherwise
• sortBy("priority") orders high > medium > low
• No test files were modified
• No new dependencies added