fix: repair all failing tests in utility library

[stooit]

Summary

Fixes all 16 previously-failing tests across the TypeScript utility library. All 60 tests now pass (0 fail), and no previously-passing tests were broken. No test files were modified and no dependencies were added.

Changes by file

• src/calculator.ts — divide now throws Error("Division by zero") instead of returning Infinity.
• src/string-utils.ts — wordCount now splits on /\s+/ (handles multiple/leading/trailing spaces); implemented truncate with word-boundary trimming, and correct handling when maxLength <= 3 (slices to maxLength rather than returning the full string).
• src/task-manager.ts — implemented the missing remove (returns boolean), update (returns boolean, Object.assign of changes), and sortBy (priority weight map, createdAt ascending, status via localeCompare).
• src/date-utils.ts — formatRelative uses Math.round instead of Math.floor for the days bucket (36h now rounds to "2 days ago").
• src/validator.ts — isEmail TLD quantifier relaxed from {2,4} to {2,} (allows TLDs like .museum); isUrl now checks only that the protocol is http/https.

Verification

• bun test → 60 pass / 0 fail.
• Changes were plan-driven, implemented, and independently reviewed for correctness beyond the test assertions.

Assumptions

• truncate's ellipsis is the literal "..." (3 chars) counted within maxLength.
• sortBy("createdAt") sorts oldest-first; unknown field falls back to status ordering.