fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Makes all 16 previously-failing tests pass while keeping the existing 44 green (60/60 pass). Fixes were limited to source files only — no test files or dependencies were touched.

Changes by file

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0.
• src/string-utils.ts — implemented truncate (word-boundary truncation with the "..." ellipsis counted toward maxLength, plus a fallback when maxLength <= 3); fixed wordCount to collapse multiple consecutive spaces.
• src/task-manager.ts — implemented remove (returns false for unknown id), update (in-place update, false for unknown id), and sortBy for createdAt (oldest first) and priority (high → medium → low).
• src/date-utils.ts — fixed off-by-one in formatRelative by rounding rather than flooring the day delta (36h now reads as "1 day ago").
• src/validator.ts — isEmail now accepts subdomains and long TLDs (e.g. .museum); isUrl rewritten to use the standard URL constructor so ports (e.g. http://localhost:3000) validate correctly.

Verification

• bun test → 60 pass, 0 fail.
• Independent review pass: all fixes confirmed correct and general (not test-narrow), no regressions.

Assumptions

• update restricts mutable fields to title/priority to match TaskManager semantics.
• sortBy copies before sorting so the internal store order is not mutated.