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>

docs/adr/0033-halkyon-todo.md

@@ -170,7 +170,10 @@ alternative was put to them, the choice is theirs.
   (Material's term) on Linux, macOS and Windows however narrow the window, and on phones
   and tablets a bottom sheet below expanded windows as before, so Learn and Train follow.
   A bottom sheet is a touch surface; rising from the foot of a desktop window it reads
-  as a phone's screen. In a narrow window the dialog keeps a smaller margin.
+  as a phone's screen. In a narrow window the dialog keeps a smaller margin. A sheet
+  that can't go through it asks `sheetShowsAsDialog` and follows the same rule: Learn's
+  conversation is a dialog as tall as the window there, nothing to drag. Train's route
+  import still opens its bottom sheet everywhere, until Train 0.1.1 is out.
 - **D12 The tabs are Inbox, Today, Upcoming, Browse; Today still opens** (amends D2's
   order, not what opens).
 - **D13 A task's sheet** names where the task is on top (its project, an icon once tasks