fix: repair 16 failing tests across utility library

[stooit]

Summary

Fixes all 16 failing tests across 5 source files in the utility library. bun test now reports 60 pass / 0 fail. No test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide throws Error("Division by zero") on a zero divisor instead of returning Infinity.
• src/string-utils.ts
  • wordCount splits on /\s+/ against the trimmed string so consecutive/mixed whitespace collapses correctly.
  • truncate implemented: returns input unchanged when within maxLength; otherwise cuts to the last word boundary and appends "...", keeping the result within maxLength. Hard-slices when maxLength <= 3.
• src/task-manager.ts — implemented remove (returns hit/miss), update (partial update, guards each field, false on unknown id), and sortBy (priority high>medium>low, status pending>in_progress>completed, createdAt oldest first; sorts a copy to avoid mutating internal order).
• src/date-utils.ts — formatRelative uses Math.round on the hour delta instead of Math.floor, fixing the day off-by-one (36h now reads "2 days ago").
• src/validator.ts
  • isEmail allows subdomains and TLDs of 2+ letters (e.g. .museum) while still rejecting non-emails.
  • isUrl allows URLs with a port (e.g. http://localhost:3000); protocol still restricted to http:/https:.

Assumptions (tests underspecify)

• truncate prefers the word-boundary rule over maximising length (matches the asserted "hello..." result).
• sortBy ties rely on V8's stable sort (insertion order preserved for equal keys); no explicit tie-break added.

Verification

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

Follow-up note

The isEmail/isUrl regexes touch input validation. A security review of the email pattern for ReDoS characteristics would be reasonable before use on untrusted input at scale.