feat(todo): dialogs on the desktop, the tabs reordered, a task's path and menu (ADR 0033, D11 to D14)

- halkyon_ui: showAdaptiveSheet shows a dialog on Linux, macOS and Windows
  at every width (bottom sheets stay on phones and tablets below expanded
  windows), so Learn and Train follow; SheetScaffold takes a heading in
  the title's place and may leave out its close button.
- Todo: the tabs are Inbox, Today, Upcoming, Browse, Today still opening;
  `serverpod start` launches the app on Linux, as it has no web build.
- A task's sheet shows where the task is (its project, an icon once tasks
  are above it, and the tasks above it, each opening what it names) and a
  ⋮ menu in place of its close button: when it was added (its UUID v7),
  Duplicate with its sub-tasks, hiding its done sub-tasks (synced in its
  payload), Close in the pane, Delete after asking. The description's icon
  stays by its first line.
- A row names its project and section at the end of its date's line; every
  menu entry has its icon.
- Learn: a studyset's path starts at the Library tab's icon, named by its
  tooltip.

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

apps/learn/halkyon_learn_flutter/lib/src/feature/studyset/widget/studyset_breadcrumb.dart

@@ -3,11 +3,14 @@ import 'dart:math' as math;
 import 'package:halkyon_learn_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_navigation.dart';
+import 'package:lucide_icons_flutter/lucide_icons.dart';
 import 'package:material_ui/material_ui.dart';
 
-/// Where a studyset sits, on one line: "Library › Japanese › Kana ›
-/// Hiragana", the steps as plain links in a muted colour, the studyset
-/// itself last, bold in the primary colour.
+/// Where a studyset sits, on one line: "📖 › Japanese › Kana › Hiragana",
+/// the library as the Library tab's icon, named by its tooltip (as Todo's
+/// path starts at its project's icon, Todo's ADR 0033, D13), the other
+/// steps as plain links in a muted colour, the studyset itself last, bold
+/// in the primary colour.
 ///
 /// Every step before it goes back up there ([goUpTo]): the screens behind
 /// a studyset are its path, so going up closes the screens above, as the
@@ -40,6 +43,9 @@ class const StudysetBreadcrumb({
   /// How high every step is, to tap.
   static const minTarget = 48.0;
 
+  /// How wide a step shown as its icon is.
+  static const _iconWidth = 32.0;
+
   /// How far before the screen's end the line ends when [trailing] ends it.
   static const trailingEndPadding = 8.0;
 
@@ -57,6 +63,7 @@ class const StudysetBreadcrumb({
       _Step(
         l10n.libraryTitle,
         enabled ? () => goUpTo(context, null, all) : null,
+        icon: LucideIcons.book,
       ),
       for (final ancestor in ancestors)
         _Step(
@@ -85,7 +92,10 @@ class const StudysetBreadcrumb({
         }
 
         final separatorWidth = widthOf('›', muted) + 2 * _separatorPadding;
-        final widths = [for (final step in steps) widthOf(step.label, muted)];
+        final widths = [
+          for (final step in steps)
+            if (step.icon == null) widthOf(step.label, muted) else _iconWidth,
+        ];
         final currentWidth = widthOf(current.content.title, active);
         const foldWidth = _Fold.width;
         // Fold the first steps away until the rest fits; at the least, the
@@ -120,13 +130,23 @@ class const StudysetBreadcrumb({
                 child: GestureDetector(
                   behavior: HitTestBehavior.opaque,
                   onTap: step.onTap,
-                  child: ConstrainedBox(
-                    constraints: const BoxConstraints(minHeight: minTarget),
-                    child: Center(
-                      widthFactor: 1,
-                      child: Text(step.label, maxLines: 1, style: muted),
+                  child: switch (step.icon) {
+                    final icon? => Tooltip(
+                      message: step.label,
+                      child: SizedBox(
+                        width: _iconWidth,
+                        height: minTarget,
+                        child: Icon(icon, size: 18, color: muted.color),
+                      ),
                     ),
-                  ),
+                    null => ConstrainedBox(
+                      constraints: const BoxConstraints(minHeight: minTarget),
+                      child: Center(
+                        widthFactor: 1,
+                        child: Text(step.label, maxLines: 1, style: muted),
+                      ),
+                    ),
+                  },
                 ),
               ),
               separator(),
@@ -160,8 +180,13 @@ class const StudysetBreadcrumb({
   }
 }
 
-/// A step of the way, and where it leads; nowhere while null.
-class const _Step(final String label, final VoidCallback? onTap);
+/// A step of the way, and where it leads; nowhere while null. One with an
+/// [icon] shows it, its [label] the tooltip, and both in the fold's menu.
+class const _Step(
+  final String label,
+  final VoidCallback? onTap, {
+  final IconData? icon,
+});
 
 /// The steps that didn't fit, behind a "…" that turns primary under the
 /// pointer, nearest first in its menu; a square target as high as a step.
@@ -184,7 +209,11 @@ class _FoldState() extends State<_Fold> {
   Widget build(BuildContext context) => MenuAnchor(
     menuChildren: [
       for (final step in widget.steps)
-        MenuItemButton(onPressed: step.onTap, child: Text(step.label)),
+        MenuItemButton(
+          onPressed: step.onTap,
+          leadingIcon: step.icon == null ? null : Icon(step.icon),
+          child: Text(step.label),
+        ),
     ],
     builder: (context, menu, _) => MouseRegion(
       cursor: SystemMouseCursors.click,