fix: repair failing utility tests and implement missing functions

[stooit]

Summary

Fixes all 16 failing tests in the utility library (was 44 pass / 16 fail, now 60 pass / 0 fail). Only src/ files were changed — no test files modified, no dependencies added.

Changes

• calculator.ts — divide now throws on division by zero instead of returning Infinity/NaN.
• string-utils.ts — wordCount splits on \s+ (handles consecutive spaces/tabs/newlines); truncate implemented with word-boundary truncation, ellipsis counting toward maxLength, returns unchanged when within limit, and handles strings shorter than the ellipsis.
• task-manager.ts — remove and update return false for unknown ids; update applies only supplied fields; sortBy supports priority (high→medium→low) and createdAt (oldest first), with deterministic tie-breaking on id.
• date-utils.ts — formatRelative off-by-one fixed: day count now rounds hours to days (36h → "2 days ago").
• validator.ts — isEmail accepts long TLDs (relaxed {2,4}→{2,}, added RFC-5321 254-char cap); isUrl accepts URLs with a port (dropped the erroneous empty-port requirement, protocol allowlist retained).

Testing

bun test → 60 pass, 0 fail (70 expect() calls). tsc --noEmit exits 0.

Assumptions

• The test suite is the source of truth for expected behaviour.
• truncate reserves 3 chars for ... only when maxLength > 3; otherwise it hard-slices.
• update treats omitted fields as "leave unchanged" (fields cleared only when explicitly present).
• sortBy breaks ties on numeric id for deterministic ordering.

Notes

• isEmail/isUrl are format checks, not deliverability/reachability checks.