feat(learn): a Viewer suggests new flashcards and notes

ADR 0044's step 6b (D48) in the app: where an Editor has Add material, a
Viewer of a shared studyset has Suggest material, offering a flashcard
or a note. The composer and the note editor open suggesting from the
start, saying that what is suggested joins the studyset once its owner
or an editor takes it in; Send suggestion asks why, and Cancel or
sending closes it. Nothing is written into the studyset: the fields go
into the suggestion alone, which goes into the Viewer's own space as
the changes they suggest do.

Whats-New: In a studyset shared with you as a viewer, Suggest material proposes a new flashcard or note to its owner and editors.
Whats-New-De: In einem Lernset, das du nur ansehen darfst, schlägst du über „Material vorschlagen“ neue Karteikarten und Notizen vor.

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/0044-shared-spaces-and-collaboration.md

@@ -381,8 +381,10 @@ member reads everything in the space.
       where it names the studyset's own files and brings none; a Viewer's
       card and note offer Suggest a change, and a shared studyset's menu its
       Suggestions, where its Owner and Editors accept or decline. New cards
-      and notes are suggested through the repository (the agent, step 6c);
-      the app offers a Viewer no button for them yet.
+      and notes are suggested through the repository (the agent, step 6c),
+      and in the app from a Viewer's Suggest material, in place of Add
+      material: the composer or note editor opens suggesting from the
+      start, Cancel or Send closing it, nothing added.
    c. The agent itself, its CLI and MCP server, on a pure-Dart core (D7, D41, D49 to
       D52).