fix: repair failing utility-library tests across 5 modules

[stooit]

Summary

Fixes all 16 failing tests in the utility library. 60/60 tests now pass, tsc --noEmit is clean, and no previously-passing tests were broken. No test files were modified and no dependencies were added.

Changes by module

• src/calculator.ts — divide now throws Error("Division by zero") on b === 0 instead of silently returning Infinity/NaN.
• src/date-utils.ts — fixed off-by-one in relative-day formatting: the day bucket used Math.floor(diffHours / 24) (36h → "1 day"); now rounds at format time so 36h correctly reads "2 days ago".
• src/string-utils.ts — implemented truncate (breaks on word boundary, ellipsis counts toward maxLength, returns unchanged when within limit). Also fixed wordCount to split on /\s+/ after trimming so runs of whitespace (incl. tabs/newlines) count as one separator.
• src/task-manager.ts — implemented remove (true/false on delete), update (applies only present keys, false if id absent), and sortBy (priority high→medium→low, status, ascending createdAt; sorts a copy so the internal Map order is untouched).
• src/validator.ts — isEmail now accepts long TLDs and subdomains (old {2,4} cap rejected .museum) while still tightening malformed domains like a..com; isUrl now accepts explicit ports (e.g. http://localhost:3000) while keeping the http/https scheme allowlist.

Assumptions

• Interpreted each function's contract from the assertions in test/ where the source lacked a spec (notably truncate word-boundary behaviour and sortBy ordering).
• sortBy("status") ordering is not asserted by any test; chose a reasonable fixed ranking.
• Kept the change set to exactly what the tests require, per the task constraint.

Reviewer notes (out-of-scope edge cases, non-blocking)

A review pass flagged edge cases the tests don't cover — left as-is to respect the "fix only what tests require" constraint:

• truncate with a negative/NaN maxLength behaves oddly (slice offset semantics); no test exercises this.
• isUrl/isEmail validate syntax only — not SSRF/injection guards. Callers using them to gate server-side requests or output should add their own allowlist/sanitisation.
• divide only guards 0, not NaN divisors.
• update treats an explicit {field: undefined} as a no-op rather than clearing the field.

Testing

60 pass
 0 fail
Ran 60 tests across 5 files.