fix(utils): repair failing tests and implement missing utilities

[stooit]

Summary

Fixes all 16 failing tests across the utility library (60/60 now pass). Only src/ files were changed — no test files and no dependencies were touched.

Changes

• calculator — divide now throws on a zero divisor instead of returning Infinity.
• string-utils — wordCount collapses runs of consecutive whitespace (trim().split(/\s+/)); truncate implemented: returns input unchanged when within maxLength, otherwise cuts at the last word boundary within budget and appends ... (ellipsis counts toward maxLength), with fallbacks for very small limits and no-space budgets.
• task-manager — implemented remove (returns true/false), update (guards unknown ids, only applies keys explicitly present), and sortBy (priority high→medium→low, status, and ascending createdAt).
• date-utils — day diff uses Math.round rather than Math.floor, so 36h correctly reports "2 days ago" (off-by-one fix).
• validator — isEmail accepts long TLDs (e.g. .museum) and rejects empty labels; isUrl accepts URLs with a port (e.g. http://localhost:3000) while still restricting the protocol to http/https.

Verification

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

Assumptions / notes

• divide throws a generic Error — the test only asserts .toThrow().
• sortBy("createdAt") gained an explicit id tiebreaker because add() stamps createdAt to the millisecond; back-to-back adds could otherwise collide and rely on sort stability.