fix: resolve failing tests and implement missing utilities

[stooit]

Summary

• calculator.divide throws on division by zero
• string-utils: implement truncate (word-boundary, ellipsis within maxLength); fix wordCount for mixed whitespace
• task-manager: implement remove, update (partial, returns false for unknown id), sortBy (priority/status/createdAt)
• date-utils.formatRelative: round days instead of floor (36h → "2 days ago")
• validator: isEmail allows long TLDs; isUrl allows explicit ports

Verification

bun test: 60 pass / 0 fail. bunx tsc --noEmit clean. No test files changed, no new dependencies.

Assumptions / notes

• divide throwing is a behaviour change for callers expecting Infinity (required by tests).
• Sort orders chosen: high>medium>low, in_progress>pending>completed, createdAt ascending.
• isUrl accepts any http(s) host incl. localhost; don't use it as an SSRF guard.