fix: make all tests pass — implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library. All 60 tests now pass (bun test), tsc --noEmit is clean, and no previously-passing tests were broken.

Changes

• src/calculator.ts — divide now throws on division by zero instead of returning Infinity/NaN.
• src/date-utils.ts — Fixed the off-by-one / rounding-overflow bug in formatRelative. Buckets are chosen after rounding, so top-of-bucket values roll up correctly (e.g. 59.6 min → "1 hour ago", 23.6 h → "1 day ago") instead of emitting "60 minutes ago".
• src/string-utils.ts — Implemented truncate (word-boundary truncation with "..." counting toward maxLength; unchanged when str.length <= maxLength). Fixed wordCount for consecutive spaces.
• src/task-manager.ts — Implemented remove, update, and sortBy (priority high→medium→low; createdAt oldest-first). remove/update return false for unknown ids.
• src/validator.ts — Broadened isEmail (long TLDs, e.g. .museum) and isUrl (host with port, e.g. http://localhost:3000).

Verification

• bun test → 60 pass / 0 fail
• tsc --noEmit clean
• A review pass caught a residual rounding-overflow bug in formatRelative, fixed before this PR.

Assumptions & scope

• No test files modified (tests treated as the spec); no dependencies added.
• Validator regexes broadened only enough to satisfy tested edge cases — kept lightweight rather than fully RFC-exhaustive to stay in scope.