fix: repair all failing tests across utility modules

[stooit]

Summary

Fixes all 16 failing tests across the 5 utility modules. Test suite goes from 44 pass / 16 fail to 60 pass / 0 fail. tsc --noEmit is clean. No test files were modified and no dependencies were added — only the 5 src/ files changed.

Changes by module

• calculator.ts — divide now throws on a zero divisor (incl. -0) instead of returning Infinity.
• string-utils.ts — wordCount splits on whitespace runs (/\s+/ on the trimmed string) so consecutive spaces/tabs/newlines no longer inflate the count. truncate implemented: returns input unchanged within limit, otherwise cuts at a word boundary within the ellipsis budget, with a hard-cut fallback for over-long single words. Guards non-finite / <= 0 maxLength to "".
• task-manager.ts — remove returns true/false on hit/miss; update returns false for unknown ids and applies only keys present in changes (partial-update semantics); sortBy returns a sorted copy with rank maps for priority/status, createdAt ascending, and a numeric-id tiebreak for deterministic ordering.
• date-utils.ts — formatRelative rounds the day magnitude, fixing the 36h boundary (now "2 days ago") and keeping past/future phrasing symmetric.
• validator.ts — isEmail accepts multi-level subdomains and 2+ char TLDs (e.g. .museum); isUrl no longer rejects valid URLs that carry a port (e.g. http://localhost:3000).

Verification

• bun test → 60 pass, 0 fail, 70 expect() calls
• tsc --noEmit → clean
• Reviewed by the review subagent; two edge-case robustness findings were folded in before commit.

Assumptions

• Kept isUrl's existing protocol allowlist (rejects ftp:/javascript:/data:).
• sortBy returns a sorted copy rather than mutating internal state.
• truncate treats non-finite / non-positive maxLength as "".