fix: Fix bugs and clean up non-idiomatic code

[spydon]

Description

A pass through the whole package that fixes the bugs I could reproduce and restructures the parts that were not idiomatic Flutter. Every bug below has a regression test.

Bugs fixed

• Toolbar icons on touch platforms: on Android and iOS with a wide screen, the icons on the right were half clipped and partly untappable, and the close button overlapped the pin icon. DashbookIcon now uses a shrink wrapped tap target.
• Device settings crashes: pressing "Reset" while "Custom device" was checked threw a null check error, and reopening the panel with a custom device active hit a dropdown assertion. Whether the device is custom is now derived from the settings instead of being kept as separate local state.
• Theme chooser crash: Dashbook.multiTheme asserted when two entries had equal ThemeData. The selected theme is now tracked by name, and an empty theme map fails with a clear assertion.
• Stale bookmark: a bookmark pointing at a removed chapter left the preview empty. It now falls back to the first chapter, also when the first story has no chapters.
• usePreviewSafeArea: it only worked inside a device frame since the device preview feature was added. It applies to the plain preview again.
• Number property: it only accepted digits, so decimals and negative numbers could not be typed.
• Edge insets and border radius properties: the values overflowed the properties panel on medium widths, the editor truncated decimals and the text controllers were leaked.
• Custom device size: the validators never ran, so typing "1" produced a one pixel wide device. Values that are out of range now show the error and are ignored, and editing the size no longer resets the rotation.
• Web: chapter links and the share button were silently disabled in WebAssembly builds since the conditional import checked dart.library.html. A malformed link hash could leave the app blank, and a deep link stacked two pages.
• Smaller ones:
  • The pin icon was invisible but still tappable on phones.
  • A failed "See code" launch threw a string and now shows a snack bar.
  • The device panel was titled "Properties" and is now "Device settings".
  • The bookmark tooltips were wrong.
  • setState could be called after dispose during loading.
  • The toolbar now scrolls when it does not fit the screen.

Cleanup

• The 280 line build method in widget.dart is split into DashbookPage, Toolbar and the theme icon widgets.
• Property editors no longer pass state through createState, most of them became stateless.
• Dead code is removed: Link, DeviceDialogButtons, the duplicated DevicePropertyScaffold, the duplicate platform file, unused breakpoints, and mockingjay together with its commented out test.
• Misspelled files and directories are renamed, and the typos in the README and the outdated code link in the example are fixed.
• sort_constructors_first, no_logic_in_create_state, only_throw_errors and prefer_function_declarations_over_variables are enabled again in analysis_options.yaml.
• The test helper for the screen size ignored the device pixel ratio, so tests asking for a 2000 wide screen really ran at 666. That is fixed.

Things that users might notice

• ControlProperty and PropertyOption now have const constructors, so users with prefer_const_constructors enabled will get new hints.
• DashbookContext.properties, DashbookContext.actions, Story.chapters and Chapter.ctx are now final.
• Chapter names in the stories list are indented with padding instead of two leading spaces.
• The "Same value to all" switch follows the theme instead of using hard coded blue and grey.

Not included

• flutter_markdown is discontinued in favour of flutter_markdown_plus, and very_good_analysis is four major versions behind. Dependency upgrades are left for a separate change.
• sliderProperty still only supports the range 0 to 1. Fixing that properly means adding min and max parameters, which is a feature.
• Chapter.widget(), Decorator.decorate() and Property.createPropertyEditor() are unchanged since they are public API.

Testing

• dart analyze --fatal-infos . and dart format . --set-exit-if-changed are clean.
• flutter test passes (74 tests), and so does flutter test --platform chrome.
• The web build of the example was driven in headless Chrome to check deep links, the properties panel, the custom device flow, the theme toggle and the share button.