feat(base,learn): suggest changes within a shared studyset

ADR 0044's step 6b (D44, D46 to D48): a Viewer, and an agent, suggest
changes within a shared studyset, which its Owner and Editors accept or
decline.

A suggestion or report is written as Explore's are, in its writer's own
synced space, with kinds of its own (space-suggestion, space-report)
and the space's id in feedback_thread's publicationId, so no synced
table changes and older apps, skipping kinds they don't know, keep
syncing. Where the studyset is private it is sealed with its key, bound
to the space; a reply to it is sealed for its thread.

Learn's server gathers a space's threads for its members alone
(feedback.threadsInSpace, NotFound for anyone else): only threads and
replies by members now, decisions only from rows in the space itself,
where the engine lets its Owner and Editors alone write; sealed
payloads pass in base64 with their key and nonce. The inbox adds the
threads its caller decides on or wrote. The module tells a host a
person's spaces and each space's members (sharedSpacesOf,
membersOfSpaces). No migration: no table changes.

In the app, a Viewer's card and note offer Suggest a change: the fields
write into the suggestion alone, sent with a message. A shared
studyset's menu lists its Suggestions, where its Owner and Editors
accept a change as a revision, or a new card or note into the
studyset, and decide in the space. A suggestion naming a file that
isn't the studyset's, or bringing files of its own, isn't taken.

Whats-New: In a studyset shared with you as a viewer, suggest a change to a card or note; its owner and editors find your suggestions under Suggestions in its menu and accept or decline them.
Whats-New-De: In einem Lernset, das du nur ansehen darfst, schlägst du Änderungen an Karteikarten und Notizen vor; Besitzer:in und Bearbeiter:innen finden sie im Menü unter „Vorschläge“ und nehmen sie an oder lehnen sie ab.

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/l10n/app_de.arb

@@ -2310,5 +2310,13 @@
   "inboxSharedOpen": "Öffnen",
   "studysetNew": "Neu",
   "shareRefusedAgentViewer": "Ein Agent kann nur Betrachter:in sein.",
-  "shareAgent": "Agent"
+  "shareAgent": "Agent",
+  "feedbackOnNewCard": "Eine neue Karteikarte",
+  "feedbackOnNewNote": "Eine neue Notiz",
+  "feedbackDecider": "Besitzer:in oder Bearbeiter:in",
+  "studysetSuggestions": "Vorschläge",
+  "suggestInSpace": "Änderung vorschlagen",
+  "suggestInSpaceIntro": "Besitzer:in und Bearbeiter:innen dieses Lernsets sehen es neben ihrer Fassung und übernehmen es oder nicht. Alle darin lesen es, mit deinem öffentlichen Profil.",
+  "suggestingNote": "Du schlägst eine Änderung vor: Nichts ändert sich, bis Besitzer:in oder Bearbeiter:innen sie übernehmen.",
+  "suggestSend": "Vorschlag senden"
 }