fix: resolve all failing tests in utility library

[stooit]

Summary

Fixes all 16 failing tests across 5 source files in the TypeScript utility library. All 60 tests now pass (previously 44 pass / 16 fail). No test files were modified and no dependencies were added.

Changes by file

• src/calculator.ts — divide now throws "Division by zero" instead of silently returning Infinity.
• src/date-utils.ts — fixed off-by-one in formatRelative: uses Math.round for day calculation so 36 hours ago correctly renders as "2 days ago".
• src/string-utils.ts — implemented truncate (word-boundary truncation, ellipsis counting toward maxLength, handling strings shorter than the ellipsis, and returning unchanged when within limit); fixed wordCount to collapse multiple consecutive spaces.
• src/task-manager.ts — implemented missing remove, update, and sortBy (priority and createdAt ordering) methods.
• src/validator.ts — fixed isEmail to accept long TLDs and subdomains; rewrote isUrl using the URL constructor so URLs with ports (e.g. http://localhost:3000) validate correctly.

Verification

• bun test → 60 pass, 0 fail
• Independent code review confirmed all fixes are correct implementations of intended behaviour, not special-casing of test inputs.

Assumptions

• The intended divide behaviour on zero is to throw (consistent with the test expecting an error).
• formatRelative uses round-half semantics for the hours/days boundary, consistent with the 36h → "2 days ago" test.
• Email/URL validators favour the permissive-but-standard approach used in utility libraries.