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_en.arb

@@ -2950,5 +2950,21 @@
   "shareRefusedAgentViewer": "An agent can only be a Viewer.",
   "@shareRefusedAgentViewer": {"description": "An agent invited or set to another role than Viewer, refused"},
   "shareAgent": "Agent",
-  "@shareAgent": {"description": "Marks a member that is an agent, a program acting for its owner"}
+  "@shareAgent": {"description": "Marks a member that is an agent, a program acting for its owner"},
+  "feedbackOnNewCard": "A new flashcard",
+  "@feedbackOnNewCard": {"description": "What a suggestion within a shared studyset proposes: a new flashcard"},
+  "feedbackOnNewNote": "A new note",
+  "@feedbackOnNewNote": {"description": "What a suggestion within a shared studyset proposes: a new note"},
+  "feedbackDecider": "Owner or editor",
+  "@feedbackDecider": {"description": "Marks a reply in a shared studyset by someone who decides on suggestions"},
+  "studysetSuggestions": "Suggestions",
+  "@studysetSuggestions": {"description": "The menu entry and sheet of a shared studyset's suggestions and reports"},
+  "suggestInSpace": "Suggest a change",
+  "@suggestInSpace": {"description": "A Viewer's button: change a card or note as a suggestion to the studyset's owner and editors"},
+  "suggestInSpaceIntro": "The owner and editors of this studyset see it beside theirs, and take it in or not. Everyone in it reads it, with your public profile.",
+  "@suggestInSpaceIntro": {"description": "Above the message of a suggestion within a shared studyset"},
+  "suggestingNote": "You're suggesting a change: nothing changes until the owner or an editor takes it in.",
+  "@suggestingNote": {"description": "The line above a card or note a Viewer changes as a suggestion"},
+  "suggestSend": "Send suggestion",
+  "@suggestSend": {"description": "Sends a Viewer's changed card or note as a suggestion"}
 }