fix: resolve failing tests across utility modules - #471

Summary

• calculator: divide throws on division by zero
• string-utils: implement truncate (word boundary, ellipsis counts toward maxLength); wordCount handles repeated whitespace
• date-utils: formatRelative rounds days (36h → "2 days ago")
• task-manager: implement remove, update, sortBy
• validator: isEmail accepts TLDs of 2+ letters; isUrl accepts ports

Verification

bun test: 60 pass, 0 fail (previously 16 failing). Reviewed; no critical/high issues.

Assumptions / notes

• No test files modified, no dependencies added.
• Minor untested asymmetry: exactly 36h in the future yields "in 1 day"; left as-is per 'fix only what tests require'.
• update ignores undefined fields, consistent with Partial<> semantics.