fix: repair 16 failing tests across utility library

[stooit]

Summary

Fixes all 16 failing tests in the utility library. Test suite now reports 60 pass, 0 fail, and tsc --noEmit is clean. No test files were modified and no dependencies were added.

Changes

File	Fix
src/calculator.ts	divide now throws Error("Division by zero") when the divisor is 0 (previously returned Infinity). Covers -0 too.
src/string-utils.ts	wordCount splits on /\s+/ against the trimmed string so consecutive/leading/trailing whitespace no longer counts as words. truncate implemented: returns input unchanged when within maxLength, otherwise cuts at the last word boundary with the ellipsis counted toward maxLength.
src/task-manager.ts	Implemented the three stubbed methods: remove (returns false for unknown id), update (partial update, false for unknown id), and sortBy for priority (high→medium→low) and createdAt (oldest first). Sorts a copy so internal order is preserved.
src/date-utils.ts	formatRelative day rounding fixed: rounds the magnitude (Math.round(Math.abs(diffHours) / 24)) so past and future intervals are symmetric — 36h resolves to "2 days ago" / "in 2 days".
src/validator.ts	isEmail allows TLDs longer than 4 chars (e.g. .museum) and subdomains. isUrl accepts a port (e.g. http://localhost:3000) while still restricting the protocol to http/https.

Verification

• bun test → 60 pass / 0 fail
• tsc --noEmit → clean
• Independent review pass caught a Math.round-on-signed-value regression in date-utils (future dates were asymmetric with past dates); this was fixed before commit.

Assumptions & notes

• The test files define the contract; fixes satisfy the tests without altering test files.
• sortBy("createdAt") relies on ES2019 stable-sort for tasks created within the same millisecond (falls back to insertion order) — safe on Bun/Node.
• isEmail/isUrl are format validators, not safety checks. isUrl accepts localhost/private hosts by design (required by the tests); if its output ever gates an outbound fetch it needs separate SSRF/open-redirect controls.