fix: implement missing utility functions and fix edge-case bugs

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library. All 60 tests now pass (0 fail); tsc --noEmit is clean. No test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide now throws on division by zero instead of returning Infinity.
• src/string-utils.ts
  • wordCount — trims and splits on \s+, collapsing consecutive/leading/trailing whitespace ("hello world" → 2).
  • truncate — implemented: returns unchanged when str.length <= maxLength; otherwise reserves 3 chars for "...", backs up to the last word boundary, and appends the ellipsis (ellipsis counts toward maxLength).
• src/task-manager.ts — implemented remove (Map delete → true/false), update (partial title/description/priority, false if not found), and sortBy (priority high>medium>low, status, createdAt oldest-first). Sort copies the values before sorting, so internal order is untouched.
• src/date-utils.ts — formatRelative day diff uses Math.round instead of Math.floor, so 36 hours renders as "2 days ago".
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum) and subdomains; isUrl accepts URLs with ports (e.g. http://localhost:3000) by validating on protocol only.

Verification

• bun test → 60 pass, 0 fail, 70 expect() calls.
• tsc --noEmit exits 0.
• A review pass empirically confirmed the truncate length invariant across edge cases and ruled out ReDoS in the new email regex.

Notes / assumptions

• update(id, { description: undefined }) intentionally no-ops rather than clearing the field — no test covers clearing, and scope was to fix only what tests require.
• Left untouched: a pre-existing hours/minutes boundary artefact in formatRelative that predates this change and is outside the failing-test scope.