fix: implement missing utility functions and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests across the TypeScript utility library. 60 pass, 0 fail after the change (was 44 pass / 16 fail), tsc --noEmit clean, no test files modified, no new dependencies.

Changes by file

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0.
• src/string-utils.ts — wordCount handles multiple consecutive spaces; truncate implemented (returns unchanged when within limit, truncates at a word boundary, "..." counts toward maxLength, handles strings shorter than the ellipsis).
• src/task-manager.ts — implemented the missing remove, update, and sortBy methods (sort by priority high→medium→low, and by createdAt oldest-first).
• src/date-utils.ts — fixed an off-by-one in formatRelative day rounding (36 hours now reports "1 day ago").
• src/validator.ts — isEmail accepts long TLDs; isUrl accepts URLs with an explicit port.

Verification

• bun test -> 60 pass, 0 fail, stable across repeated runs.
• Independent code review: implementations are general (not tuned to specific assertions), no regressions, no test files touched.

Assumptions

• Scope limited to "fix only what the tests require," per the task. Untested edge cases (e.g. truncate with a negative maxLength) were left as-is to avoid changing behaviour beyond the test contract.