fix: repair failing utility tests and implement missing functions

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library and implements the missing/stubbed functions. All 60 tests now pass (0 failures). 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 (was returning Infinity).
src/string-utils.ts	wordCount collapses consecutive whitespace (trim().split(/\s+/)); implemented truncate with word-boundary truncation where the "..." ellipsis counts toward maxLength.
src/task-manager.ts	Implemented remove (returns false for unknown id), update (applies changes, false for unknown id), and sortBy (priority high→low, createdAt oldest-first).
src/validator.ts	isEmail accepts long TLDs (e.g. .museum); isUrl accepts URLs with an explicit port (e.g. http://localhost:3000).
src/date-utils.ts	formatRelative rounds the day count instead of flooring, fixing the off-by-one (36h → "2 days ago").

Also removed now-stale // BUG: comments that described the just-fixed defects.

Testing

bun test → 60 pass / 0 fail (70 assertions).

Assumptions

• sortBy("priority") orders high→medium→low and sortBy("createdAt") orders oldest-first, matching the tests.
• truncate treats the 3-char ellipsis as part of the maxLength budget.
• Per the "fix only what the tests require" constraint, untested edge cases were intentionally left unchanged to avoid scope creep.