fix: repair all failing tests across the utility library

[stooit]

Summary

Fixes all 16 failing tests across the 5 source modules so the full suite passes (60 pass / 0 fail, tsc --noEmit clean). No test files modified, no dependencies added.

Changes

• src/calculator.ts — divide throws Error("Division by zero") when b === 0 instead of silently returning Infinity.
• src/string-utils.ts
  • Implemented truncate: unchanged when within maxLength; otherwise reserves 3 chars for "..." (ellipsis counts toward the limit), cuts at the last word boundary with a hard-cut fallback, trims trailing whitespace. Hardened against negative maxLength (clamped to 0) and leading-whitespace inputs.
  • wordCount trims then splits on /\s+/, collapsing whitespace runs.
• src/task-manager.ts — Implemented remove (Map.delete), update (returns false for unknown ids; only assigns fields present in changes), and sortBy (sorts a copy via typed Record<Priority,number>/Record<Status,number> rank maps).
• src/date-utils.ts — diffDays uses Math.round(Math.abs(diffHours) / 24), fixing the off-by-one that bucketed 24–47h as "1 day".
• src/validator.ts — isEmail TLD quantifier {2,4} → {2,} (accepts long TLDs); isUrl drops the port === "" condition so http://localhost:3000 validates, retaining http/https allowlisting.

Assumptions

• update treats omitted keys as "leave untouched"; clearing description via { description: undefined } is intentionally a no-op.
• trimStart() deliberately avoided in truncate to keep within-limit/whitespace-only inputs unchanged.

Testing

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

Reviewer notes (out of scope, non-blocking)

• isEmail is a format check only, not a deliverability/authorisation control.
• isUrl allowlists the scheme but not the host — SSRF risk if a caller server-side-fetches a user-supplied URL.