fix: resolve 16 failing tests across utility modules

[stooit]

Summary

Fixes all 16 previously-failing tests in the utility library. Test suite now reports 60 pass / 0 fail (was 44 pass / 16 fail). Only the five src/ files were modified — no test files touched, no dependencies added.

Changes

• src/calculator.ts — divide now throws on a zero divisor instead of returning Infinity (fail-closed on invalid input).
• src/string-utils.ts — implemented truncate (word-boundary aware, never exceeds maxLength); fixed wordCount to split on /\s+/ after trimming so runs of whitespace and whitespace-only input count correctly.
• src/task-manager.ts — implemented remove (returns true/false), update (partial update, returns false for unknown id, omitted keys never clobber existing values), and sortBy (priority/status rank maps, chronological createdAt; sorts a copy, non-mutating).
• src/date-utils.ts — fixed off-by-one in formatRelative: day bucket now uses Math.round(abs(diffHours)/24) instead of a pre-sign floor, so 36 hours correctly reads "2 days ago".
• src/validator.ts — isEmail accepts long TLDs (example.museum) via {2,}; isUrl accepts host:port URLs (http://localhost:3000) by dropping the erroneous empty-port rejection, while still restricting the scheme allowlist to http/https.

Verification

• bun test → 60 pass / 0 fail (70 expect() calls)
• tsc --noEmit clean
• No test files modified; no dependencies added

Assumptions / notes

• isEmail is a pragmatic format-validation regex, not RFC 5322-complete — it should not be treated as proof an address is deliverable. If these validators gate anything security-relevant, delivery confirmation is the real control.