feat(learn): the assistant's conversation a dialog on the desktop and in expanded windows (Todo's ADR 0033, D11)

- halkyon_ui's sheetShowsAsDialog says whether showAdaptiveSheet shows
  a dialog here, for a sheet of its own that can't go through it.
- Learn's conversation, a half-height sheet dragged up, asks it: where
  sheets are dialogs it is a dialog up to 720 wide and as tall as the
  window, with no handle to drag.

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/assistant/widget/conversation_sheet.dart

@@ -25,7 +25,9 @@ import 'package:material_ui/material_ui.dart';
 
 /// Opens a conversation with [choice] in [scope] for [task] in a sheet over
 /// the screen it is asked on (ADR 0021, D47): at half height, dragged up to
-/// the full, with follow-up questions at its foot. Asks [question] with
+/// the full, with follow-up questions at its foot; where halkyon_ui's
+/// sheets are dialogs (from expanded windows on, and on the desktop), a
+/// dialog as tall as the window allows. Asks [question] with
 /// [chosen] at once where given, a new conversation called [title] where
 /// given, or continues the kept conversation [conversationId]. [choice]
 /// null shows a kept conversation whose assistant has gone, which can't go
@@ -66,15 +68,65 @@ Future<void> showConversation(
       await cubit.ask(question, chosen);
     }
   }());
+  // Closing the sheet closes the cubit, which stops an answer on its way.
+  Widget provided(Widget child) =>
+      BlocProvider(create: (context) => cubit, child: child);
+  // A dialog where halkyon_ui's sheets are (Todo's ADR 0033, D11): from
+  // expanded windows on, and on the desktop at every width.
+  if (sheetShowsAsDialog(context)) {
+    return showDialog<void>(
+      context: context,
+      builder: (context) => provided(
+        _ConversationDialog(name: name, scope: scope, choice: choice),
+      ),
+    );
+  }
   return showModalBottomSheet<void>(
     context: context,
     isScrollControlled: true,
     useSafeArea: true,
-    builder: (context) => BlocProvider(
-      // Closing the sheet closes the cubit, which stops an answer on its
-      // way.
-      create: (context) => cubit,
-      child: _ConversationSheet(name: name, scope: scope, choice: choice),
+    builder: (context) =>
+        provided(_ConversationSheet(name: name, scope: scope, choice: choice)),
+  );
+}
+
+/// The conversation as a dialog, as tall as the window lets it, up to a
+/// reading width: nothing to drag.
+class const _ConversationDialog({
+  required final String name,
+  required final AssistantScope scope,
+  required final AssistantChoice? choice,
+}) extends StatefulWidget {
+  @override
+  State<_ConversationDialog> createState() => _ConversationDialogState();
+}
+
+class _ConversationDialogState() extends State<_ConversationDialog> {
+  final _controller = ScrollController();
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) => Dialog(
+    insetPadding: context.windowSize.isAtLeastMedium
+        ? const EdgeInsets.symmetric(horizontal: 40, vertical: 32)
+        : const EdgeInsets.symmetric(horizontal: 16, vertical: 24),
+    clipBehavior: Clip.antiAlias,
+    child: ConstrainedBox(
+      constraints: const BoxConstraints(maxWidth: 720),
+      child: SizedBox.expand(
+        child: _ConversationView(
+          controller: _controller,
+          name: widget.name,
+          scope: widget.scope,
+          choice: widget.choice,
+          handle: false,
+        ),
+      ),
     ),
   );
 }
@@ -99,6 +151,7 @@ class const _ConversationSheet({
         name: name,
         scope: scope,
         choice: choice,
+        handle: true,
       ),
     ),
   );
@@ -112,6 +165,9 @@ class const _ConversationView({
   /// Who answers; null for an assistant no longer offered, which can't go
   /// on.
   required final AssistantChoice? choice,
+
+  /// Whether a handle to drag it by sits on top: in a bottom sheet.
+  required final bool handle,
 }) extends StatelessWidget {
   /// How close to its end the conversation counts as followed: closer, it
   /// follows the answer as it grows.
@@ -141,7 +197,7 @@ class const _ConversationView({
               controller: controller,
               slivers: [
                 PinnedHeaderSliver(
-                  child: _Header(name: name, scope: scope),
+                  child: _Header(name: name, scope: scope, handle: handle),
                 ),
                 if (!state.keeps)
                   SliverToBoxAdapter(
@@ -202,11 +258,12 @@ class const _ConversationView({
   }
 }
 
-/// The handle to drag the sheet by, who answers and how far it looks, and
-/// Close.
+/// The handle to drag the sheet by (where [handle]), who answers and how
+/// far it looks, and Close.
 class const _Header({
   required final String name,
   required final AssistantScope scope,
+  required final bool handle,
 }) extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
@@ -217,19 +274,20 @@ class const _Header({
       child: Column(
         mainAxisSize: MainAxisSize.min,
         children: [
-          Padding(
-            padding: const EdgeInsets.only(top: 12),
-            child: Container(
-              width: 32,
-              height: 4,
-              decoration: BoxDecoration(
-                color: theme.colorScheme.onSurfaceVariant.withValues(
-                  alpha: 0.4,
+          if (handle)
+            Padding(
+              padding: const EdgeInsets.only(top: 12),
+              child: Container(
+                width: 32,
+                height: 4,
+                decoration: BoxDecoration(
+                  color: theme.colorScheme.onSurfaceVariant.withValues(
+                    alpha: 0.4,
+                  ),
+                  borderRadius: BorderRadius.circular(2),
                 ),
-                borderRadius: BorderRadius.circular(2),
               ),
             ),
-          ),
           Padding(
             padding: const EdgeInsets.fromLTRB(16, 4, 4, 4),
             child: Row(