fix: make all failing tests pass

[stooit]

Summary

Fixes all 16 failing tests across the 5 utility modules and implements the missing functionality. Test suite went from 44 pass / 16 fail → 60 pass / 0 fail (bun test). tsc --noEmit is clean.

Changes by file

• src/calculator.ts — divide now throws "Division by zero" when the divisor is 0 (was returning Infinity).
• src/string-utils.ts — implemented truncate (word-boundary truncation with "..." counting toward maxLength, unchanged when within limit); fixed wordCount to collapse consecutive whitespace via trim().split(/\s+/).
• src/task-manager.ts — implemented remove (Map.delete boolean semantics), update (per-field !== undefined guards), and sortBy (priority high→medium→low and createdAt oldest-first, sorting a copy).
• src/date-utils.ts — fixed off-by-one in formatRelative day bucket (Math.floor → Math.round); e.g. 36h now reads "2 days ago".
• src/validator.ts — isEmail TLD length {2,4} → {2,} (accepts long TLDs); isUrl dropped the erroneous port === "" check that rejected all URLs with a port, protocol allow-list (http/https only) retained.

Verification

• bun test → 60 pass / 0 fail
• bunx tsc --noEmit → clean
• No test files modified; no dependencies added; package.json unchanged.

Assumptions

• Fixed only what the tests require, per task constraints.
• sortBy("status") ordering (in_progress → pending → completed) is an untested judgement call to satisfy the union type; nothing depends on it.

Non-blocking review notes (out of test scope, left as-is)

A review flagged three defensive-hardening items not covered by tests and not reachable from current callers: truncate with negative maxLength could exceed the limit; sortBy has no default arm; update can't clear an optional description. Left to honour the "fix only what the tests require" constraint.