fix: resolve all failing tests in utility library

[stooit]

Summary

Fixes all 16 failing tests across 5 utility modules. Suite now passes 60/60 (0 failures). No test files were modified and no dependencies were added.

Changes

• calculator.ts — divide now throws on division by zero instead of returning Infinity.
• date-utils.ts — formatRelative rounds hours→days (Math.round) fixing an off-by-one (36h now reports "2 days ago").
• string-utils.ts — wordCount handles multiple consecutive spaces; truncate implemented (within-limit passthrough, word-boundary truncation, ellipsis counted toward maxLength, short-string handling).
• task-manager.ts — implemented remove (returns false for unknown id), update (title/priority, false for unknown id), and sortBy (priority high→medium→low, createdAt oldest-first).
• validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts URLs with a port.

Verification

bun test → 60 pass, 0 fail.

Assumptions

• Expected behaviour and method signatures were derived from the existing tests; only source changes required to satisfy them were made.