feat(studyset): a studyset's marker says what the instance sees of it (ADR 0019)

The developer's decision of 21 Oct, on the library's cards and the
studyset's Share in its action row alike:
- a lock: private, end-to-end encrypted;
- an open lock: readable by the instance, not published;
- a globe: published.

The lock and the open lock open one dialog saying what that means and
changing it (Make non-private; Make private, or Publish…, which opens the
Publish sheet, while it isn't private); a topic's dialog says its privacy
is its tree's. The globe opens the Publish sheet, whose status for a
studyset not published now says so ("Not published") with an open lock,
as "Private" means something else now. Texts in en and de.

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

@@ -1883,7 +1883,7 @@
   "studysetSharePrivate": "Share: private",
   "studysetSharePublished": "Share: published",
   "publishTitle": "Publish",
-  "publishPrivate": "Private · only you can see it",
+  "publishPrivate": "Not published · only you see it in Learn",
   "publishPublished": "Published · v{version} · {date}",
   "@publishPublished": {"placeholders": {"version": {"type": "int"}, "date": {"type": "String"}}},
   "publishIntro": "A release is this studyset with everything inside it, as it is now. Others can find it, learn from it and copy it. Each release stays as it was; later changes go into the next one.",
@@ -2445,5 +2445,15 @@
   "libraryOfferPrivateBody": "New studysets are private: end-to-end encrypted, so only your devices can read them. Those you made before stay readable by the instance until you make them private too. Copies in the instance's backups stay readable until those expire.",
   "libraryOfferPrivateAction": "Make all private",
   "libraryOfferPrivateLater": "Not now",
-  "exportReadableNote": "The archive isn't encrypted: whoever has the file can read it."
+  "exportReadableNote": "The archive isn't encrypted: whoever has the file can read it.",
+  "studysetVisibilityReadable": "Not private, not published",
+  "studysetShareReadable": "Share: not private",
+  "studysetPrivacyPrivateTitle": "“{title}” is private",
+  "@studysetPrivacyPrivateTitle": {"placeholders": {"title": {"type": "String"}}},
+  "studysetPrivacyPrivateBody": "Everything in it is end-to-end encrypted: only your devices can read it. The instance can't publish it, nor show it to Claude. Making it non-private decrypts it and syncs it again, so the instance can read it.",
+  "studysetPrivacyReadableTitle": "“{title}” isn't private",
+  "@studysetPrivacyReadableTitle": {"placeholders": {"title": {"type": "String"}}},
+  "studysetPrivacyReadableBody": "The instance can read it: you can publish it, and Claude can read it through the connector. 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…"
 }