fix: repair failing tests and implement missing utility functions

[stooit]

Summary

Fixes all 16 previously-failing tests (now 60 pass / 0 fail) across the TypeScript utility library, implementing missing functionality and correcting edge-case bugs. No test files modified, no dependencies added.

Changes by file

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0 (also catches -0) instead of returning Infinity.
• src/string-utils.ts
  • wordCount — splits on /\s+/ after trimming, so consecutive spaces/tabs/newlines collapse correctly.
  • truncate — implemented: returns the string unchanged when length <= maxLength; otherwise cuts at a word boundary with "..." counting toward maxLength, falling back to a hard character cut when there's no space. Guards maxLength <= 0 (returns "") and preserves content when the boundary lands in leading whitespace.
• src/task-manager.ts — implemented remove (delete + boolean), update (partial update, only applies present keys), and sortBy (priority / status / createdAt ordering on a copied array).
• src/date-utils.ts — formatRelative day calculation fixed: Math.round(Math.abs(diffHours) / 24), correcting both the past-date off-by-one and a signed-rounding regression that would have broken future dates >=24h.
• src/validator.ts — isEmail accepts TLDs longer than 4 chars ({2,}); isUrl no longer rejects URLs with an explicit port (e.g. http://localhost:3000) while still rejecting non-http(s) schemes.

Verification

• bun test -> 60 pass / 0 fail
• bunx tsc --noEmit -> clean
• Only the 5 src/ files changed; no test files, no dependency changes.

Review notes / assumptions

• A review pass caught a signed-rounding regression in date-utils (future dates >=24h) that the test suite doesn't cover; fixed proactively.
• truncate hardened against maxLength <= 0 and leading-whitespace inputs beyond what the tests assert, to avoid returning bare "..." or stray fragments.
• The sortBy status ordering (in_progress > pending > completed) is not spec'd beyond the tests — chosen to satisfy the test and left as a reasonable convention.
• isEmail remains a permissive structural regex (satisfies tests; not a security-grade validator). Not hardened per the "fix only what tests require" constraint.