fix: resolve all failing tests in utility library

[stooit]

Summary

Fixes all 16 failing tests across 5 utility modules. The full suite now passes (60 pass / 0 fail). No test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide now throws on division by zero instead of returning Infinity.
• src/string-utils.ts — wordCount correctly handles multiple consecutive spaces; truncate implemented (unchanged within limit, word-boundary cut, ellipsis counts toward maxLength, handles strings shorter than the ellipsis).
• src/task-manager.ts — added missing remove, update, and sortBy (priority high>medium>low, createdAt oldest-first) methods.
• src/date-utils.ts — fixed off-by-one in formatRelative (now uses Math.round, so 36h → "2 days ago").
• src/validator.ts — isEmail accepts long TLDs; isUrl accepts URLs with a port.

Testing

bun test → 60 pass, 0 fail, 70 expect() calls.

Review notes (non-blocking, outside test scope)

• truncate: a maxLength <= ellipsis length returns a bare slice without ellipsis — defensible for degenerate input, spec-unspecified.
• formatRelative: hours branch can report "24 hours ago" at the 24h boundary — pre-existing, not introduced here.

Assumptions

• Tests are the authoritative spec; source was changed to satisfy them.
• Existing function signatures and the public API shape were preserved.