fix: implement missing utilities and fix edge-case bugs (all tests passing)

[stooit]

Summary

Makes the full test suite pass — 60 pass, 0 fail (was 44 pass / 16 fail). No test files were modified and no dependencies were added.

Fixes by file

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0 instead of returning Infinity/NaN.
• src/string-utils.ts
  • wordCount trims and splits on \s+, so leading/trailing/consecutive whitespace no longer inflates the count.
  • Implemented truncate (was a not-implemented TODO): returns the string unchanged when length <= maxLength; otherwise cuts at the last word boundary and appends "...", with the ellipsis counting toward maxLength.
• src/task-manager.ts — implemented remove (returns false for unknown ids), update (per-field undefined guards so only supplied fields change; false for unknown ids), and sortBy (priority high→medium→low, status rank, and createdAt ascending / oldest-first).
• src/date-utils.ts — fixed off-by-one in formatRelative day calculation (Math.floor → rounded absolute hours), so exactly 36 hours reports "2 days ago" rather than "1 day ago".
• src/validator.ts — isEmail accepts arbitrary subdomain depth and 2+ char TLDs (e.g. .museum); isUrl accepts hosts with explicit ports (e.g. http://localhost:3000), still restricted to http:/https:.

Verification

bun test -> 60 pass, 0 fail across 5 files. tsc --noEmit clean.

Assumptions & notes

• Behaviour was implemented to match the intent in the code comments/TODOs and the read-only test expectations.
• Security note (not covered by tests, behaviour left as the tests require): isUrl now accepts any host including internal addresses (localhost, link-local). If this validator is ever used to gate a server-side fetch, it would need an explicit host/IP allowlist to avoid becoming an SSRF vector. isEmail remains a syntax check only, not input sanitisation.