fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests across the utility library. All 60 tests now pass (0 fail), and no previously-passing tests were broken. Only src/ files were changed — test files are untouched 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).
src/string-utils.ts	Implemented truncate — truncates at a word boundary with "..." counting toward maxLength, returns the string unchanged when within the limit. Fixed wordCount to split on /\s+/ so consecutive spaces don't inflate the count.
src/task-manager.ts	Implemented the missing/incomplete remove(), update(), and sortBy() methods (remove/update return false for unknown ids; sortBy supports priority high→medium→low and createdAt oldest-first).
src/date-utils.ts	Fixed off-by-one in formatRelative — uses Math.round instead of Math.floor, so 36h ago correctly reads "2 days ago".
src/validator.ts	Fixed isEmail to accept long TLDs and isUrl to accept URLs with a port.

Also removed two now-inaccurate // BUG: comments in string-utils.ts and date-utils.ts that described the old broken behaviour.

Verification

• bun test → 60 pass / 0 fail (70 expect() calls).
• Independent review confirmed the implementations are correct and don't break other functions' contracts.

Assumptions

• Followed the behaviour specified by the existing tests and inline TODOs; where a TODO described the intended contract (e.g. ellipsis counting toward maxLength), that contract was implemented as written.