Halkyon's shared packages

A pub workspace of the Dart packages Halkyon's apps share: halkyon_ui,
Halkyon's look (the theme, fonts, layout and shared widgets, with texts
in English and German), halkyon_core's low-level primitives and
halkyon_interop's cross-app contracts. Dart 3.13 with LeanCode's lints,
configured once at the root; apps take a package by git, pinned to a
signed commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/lib/src/layout/readable_width.dart

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+/// The widest body text should run: about 720 dp, which keeps lines within
+/// the 60 to 80 characters that read most easily.
+const double readableContentMaxWidth = 720;

packages/halkyon_ui/lib/src/layout/window_size.dart

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+import 'package:material_ui/material_ui.dart';
+
+/// Material 3's window size classes, by width in logical pixels: compact
+/// below 600, medium to 840, expanded to 1200, large to 1600, extra large
+/// beyond.
+///
+/// Layouts branch on two of the lines: from medium on, navigation is a rail
+/// at the side instead of a bar at the bottom; from expanded on, the layout
+/// is a desktop one (sheets open as dialogs, a screen may show two
+/// columns). Expanded rather than medium for the latter because medium
+/// windows are mostly tablets, used by touch, and expanded ones mostly
+/// desktops.
+enum WindowSize {
+  compact,
+  medium,
+  expanded,
+  large,
+  extraLarge;
+
+  /// The class of a window [width] wide.
+  factory fromWidth(double width) => switch (width) {
+    < 600 => compact,
+    < 840 => medium,
+    < 1200 => expanded,
+    < 1600 => large,
+    _ => extraLarge,
+  };
+
+  /// Whether navigation is a rail rather than a bottom bar.
+  bool get isAtLeastMedium => index >= medium.index;
+
+  /// Whether the layout is a desktop one.
+  bool get isAtLeastExpanded => index >= expanded.index;
+}
+
+/// `context.windowSize`: the class of the window [BuildContext] is in.
+extension WindowSizeOf on BuildContext {
+  /// Rebuilds the caller when the window's width changes.
+  WindowSize get windowSize =>
+      WindowSize.fromWidth(MediaQuery.sizeOf(this).width);
+}