fix: resolve all failing tests in utility library

[stooit]

Summary

Fixes all 16 previously-failing tests across the TypeScript utility library. The full suite now passes 60/60 (was 44 pass / 16 fail). No test files were modified and no dependencies were added — only source fixes required by the tests.

Changes by file

• src/calculator.ts — divide now throws on division by zero instead of returning Infinity.
• src/date-utils.ts — formatRelative uses Math.round (was Math.floor) for the hours→days conversion, fixing the off-by-one at the 36-hour boundary (now "2 days ago").
• src/string-utils.ts — implemented truncate (respects word boundaries, counts the ellipsis toward maxLength, handles strings shorter than the ellipsis) and fixed wordCount to split on \s+ so consecutive spaces don't inflate the count.
• src/task-manager.ts — implemented the missing/incomplete remove, update, and sortBy methods (sortBy orders priority high>medium>low and createdAt oldest-first; remove/update return false for unknown ids).
• src/validator.ts — isEmail accepts subdomains and long TLDs; isUrl accepts URLs with ports. Also refreshed a stale JSDoc comment.

Verification

• bun test → 60 pass, 0 fail, 70 expect() calls.
• Reviewed by the review subagent: fixes are general-purpose (not hard-coded to test inputs), no regressions.

Assumptions

• The intended behaviour was inferred from the existing (unmodified) tests, which were treated as the source of truth per the task brief.