fix: resolve failing tests across utility modules

[stooit]

Summary

Fixes all 16 failing tests (now 60/60 pass, tsc --noEmit clean). No test files or dependencies changed.

• calculator: divide throws on zero divisor.
• date-utils: day diff uses Math.round (consistent with minute/hour branches; 36h → "2 days ago").
• string-utils: implemented truncate (word-boundary aware, ellipsis, never exceeds maxLength); wordCount splits on /\s+/.
• task-manager: implemented remove, update (partial, returns false for unknown id), sortBy (priority/status/createdAt).
• validator: isEmail allows long TLDs (plus 254-char length bound); isUrl accepts explicit ports.

Assumptions

• sortBy("status") order is pending → in_progress → completed (untested; not specified).
• isEmail 254-char cap added defensively (RFC 5321), not required by tests.