fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests across the utility library. Final result: 60 pass / 0 fail, tsc --noEmit clean. No test files modified, no dependencies added.

Changes

• src/calculator.ts — divide throws on division by zero instead of returning a poisoned Infinity/NaN.
• src/string-utils.ts — wordCount splits on \s+ (collapses runs of spaces/tabs/newlines); truncate implemented — reserves 3 chars for "...", cuts at a word boundary (falling back to a hard cut when the boundary would lose too much), never exceeds maxLength, handles maxLength <= 3 and negative values.
• src/task-manager.ts — implemented remove (returns boolean), update (returns false for unknown id, only overwrites explicitly-supplied fields), and sortBy (priority high→medium→low, createdAt oldest-first, status ordering) with exhaustive union handling.
• src/date-utils.ts — formatRelative off-by-one fixed: 36 hours now reads "2 days ago"; symmetric for future dates.
• src/validator.ts — isEmail TLD bound widened to 2–63 chars (accepts .museum); isUrl no longer rejects URLs with an explicit port (http://localhost:3000), keeps http/https scheme allowlist.

Assumptions & notes

• Fixed only what the tests require, per the task constraints.
• isEmail/isUrl are format checks only — not deliverability or SSRF/injection defences. If isUrl is ever used to vet server-fetched URLs, loopback/private hosts (now accepted for the test) would need a call-site allowlist. Flagged for a follow-up rather than changed here.
• Pre-existing 23.6h → "24 hours ago" boundary and sortBy returning live Task references were left as-is, out of scope for this fix.

Test plan

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