fix: resolve failing tests and implement missing utilities - #458

Summary

Makes all 60 tests pass (previously 44 pass / 16 fail). tsc --noEmit is clean. No test files were changed and no dependencies were added.

• calculator: divide throws Error("Division by zero") when b === 0.
• date-utils: formatRelative rounds days with Math.round(Math.abs(hours)/24), so 36h gives "2 days ago". Past and future dates now round the same way.
• validator: isEmail accepts TLDs of 2 or more characters (it used to cap them at 4). isUrl no longer rejects URLs with a port.
• string-utils: wordCount splits on runs of whitespace. truncate is implemented: it cuts at a word boundary and the ellipsis counts toward maxLength.
• task-manager: implemented remove, update and sortBy (priority, createdAt, status).
• Removed out-of-date BUG/TODO comments.

Assumptions

• sortBy('status') puts pending first, then in_progress, then completed. No test covers this order.
• truncate with maxLength <= 3 returns the ellipsis cut down to maxLength. A single word longer than the limit is cut mid-word.
• update ignores fields left undefined, so update can't clear a description.

Known limitations (not changed, out of scope)

• isUrl only checks the protocol, so URLs containing credentials still pass. Don't use it as a security control.
• isEmail accepts some malformed addresses (e.g. a@b..com).
• Times between 23.5h and 24h show as "24 hours ago".