Chap 6 made consistent with previous chap; and follow Dart Style Guide

- Chap 6 `index.html`: some unnecessary differences with chapter 5’s `index.html` have been eliminated.
- Added types on public class members to follow the Dart Style Guide.
- Removed whitespace from chap 5 files to eliminate unnecessary differences with chap 6 files.
- Minor edit to chap 3 & 4 `index.html` to only show “Recipe Details” heading when a recipe is being shown. This is more consistent with the app behavior of chap 6 (which renders the recipe details via a completely self-contained view).