fix: repair failing tests and implement missing utility functionality

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library and implements missing functionality. All 60 tests now pass (0 fail), with no changes to test files and no new dependencies.

Changes by file

• src/calculator.ts — divide now throws Error("Division by zero") instead of returning Infinity.
• src/date-utils.ts — fixed off-by-one in formatRelative: uses Math.round on hours→days so 36h ago reads "2 days ago". Removed stale BUG comment.
• src/string-utils.ts — implemented wordCount handling of multiple consecutive spaces; implemented word-boundary truncate with an ellipsis-aware length budget (output always ≤ maxLength, including small maxLength).
• src/task-manager.ts — implemented the missing remove, update, and sortBy (priority + createdAt) methods.
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts URLs with ports (e.g. http://localhost:3000).

Verification

• bun test → 60 pass / 0 fail.
• Reviewed by the review subagent; one non-blocking truncate edge case (maxLength < ellipsis length) was hardened.

Assumptions

• formatRelative uses rounding (not floor) per the 36-hour test expectation.
• divide throws only on exact b === 0 (strict).
• sortBy priority order is high > medium > low; createdAt sorts oldest first, matching the tests.

Constraints honoured

• No test files modified.
• No new dependencies added.