feat(learn): a studyset is made private in Edit, no longer from its options

The developer's review of 3 Nov 2026: Make private and Make non-private
leave a studyset's options (its card's menu, its screen's); a top-level
studyset's Edit has a Visibility section instead, its switch making it
private, or non-private, at once once the person agreed in the dialog as
before (ADR 0019), not on Save, and what that means behind the question
mark. A topic's Edit offers none, its privacy being its tree's. The lock on
a studyset's card still explains its privacy and keeps its own button.

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

@@ -2129,5 +2129,7 @@
   "studysetPrivacyReadableTitle": "„{title}“ ist nicht privat",
   "studysetPrivacyReadableBody": "Die Instanz kann es lesen: Du kannst es veröffentlichen. Privat wird es auf deinen Geräten Ende-zu-Ende-verschlüsselt; Kopien in den Sicherungen der Instanz bleiben lesbar, bis diese ablaufen.",
   "studysetPrivacyTopic": "Seine Privatsphäre ist die des Lernsets, in dem es liegt.",
-  "studysetPrivacyPublish": "Veröffentlichen …"
+  "studysetPrivacyPublish": "Veröffentlichen …",
+  "studysetSheetVisibility": "Sichtbarkeit",
+  "studysetSheetPrivate": "Privat"
 }