fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 previously failing tests across the utility library (now 60 pass / 0 fail). Only source files were modified — no test files touched, no dependencies added.

Changes

• calculator — divide now throws on division by zero instead of silently returning Infinity.
• string-utils — wordCount collapses consecutive whitespace (splits on /\s+/); implemented truncate to cut at a word boundary with "..." counting toward maxLength, returning the string unchanged when within limit.
• task-manager — implemented the previously unimplemented remove, update, and sortBy methods (priority: high > medium > low; createdAt: oldest first; status ordering).
• date-utils — fixed off-by-one in formatRelative (Math.round instead of Math.floor); 36 hours ago now reads "2 days ago".
• validator — isEmail accepts long alphabetic TLDs (e.g. .museum); isUrl accepts URLs with an explicit port (e.g. http://localhost:3000).

Testing

bun test → 60 pass, 0 fail. tsc --noEmit clean.

Assumptions

• truncate hard-slices when maxLength <= 3 (no room for content + ellipsis), and only walks back to a word boundary when the cut lands mid-word.
• update guards each field against undefined so a partial change object cannot blank out existing values.
• sortBy relies on Array.prototype.sort stability to preserve insertion order for equal keys.

Note (out of test scope, not changed)

isEmail is a syntactic check only — it does not prove deliverability or ownership. If any caller gates account creation on it, that should be backed by a confirmation-link flow.