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_en.arb

@@ -2545,5 +2545,9 @@
   "@studysetPrivacyReadableTitle": {"placeholders": {"title": {"type": "String"}}},
   "studysetPrivacyReadableBody": "The instance can read it: you can publish it. Making it private encrypts it end to end on your devices; copies in the instance's backups stay readable until those expire.",
   "studysetPrivacyTopic": "Its privacy is that of the studyset it's in.",
-  "studysetPrivacyPublish": "Publish…"
+  "studysetPrivacyPublish": "Publish…",
+  "studysetSheetVisibility": "Visibility",
+  "@studysetSheetVisibility": {"description": "Heading in a top-level studyset's Edit sheet above its privacy switch (ADR 0019)."},
+  "studysetSheetPrivate": "Private",
+  "@studysetSheetPrivate": {"description": "The switch making a top-level studyset private (end-to-end encrypted) in its Edit sheet."}
 }