fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests (now 60 pass / 0 fail) by implementing missing functionality and correcting edge-case bugs across the utility library. No test files were modified and no dependencies were added.

Fixes

• src/calculator.ts — divide returned Infinity for a zero divisor; now throws Error("Division by zero") (fail-closed on invalid input).
• src/string-utils.ts
  • truncate — implemented: returns unchanged when str.length <= maxLength, otherwise reserves 3 chars for "...", backtracks to the last word boundary, trims trailing whitespace, and appends the ellipsis. Guards maxLength <= 3 so the result never exceeds maxLength.
  • wordCount — was split(" ") (counted empty strings from consecutive spaces). Now trims and splits on /\s+/, also handling tabs/newlines.
• src/date-utils.ts — formatRelative used Math.floor(diffHours / 24), so 12h–47h all rendered "1 day ago". Switched to Math.round; 36h now yields "2 days ago".
• src/validator.ts
  • isEmail — regex rejected long TLDs (e.g. .museum); replaced with a pattern allowing dot-separated labels and a 2+ letter TLD.
  • isUrl — removed the port === "" clause that rejected URLs carrying an explicit port; scheme allowlist (http/https) remains the control.
• src/task-manager.ts — implemented remove (delegates to Map.delete), update (returns false for unknown id; applies only present keys), and sortBy (priority/status rank maps + numeric createdAt comparator).

Testing

bun test → 60 pass / 0 fail.

Assumptions / notes

• Behaviour for each fix follows the TODO comments and test expectations in the source.
• sortBy/list hand out references to the live Task objects (no defensive copy). Tests don't exercise mutation of stored state, so this was left as-is, but worth noting if consumed across trust boundaries.