docs(store): say studysets are the learners' own and instances anyone's

The Snap Store and F-Droid listings now say what the demo video says:
every studyset is made by its learners (build one for any subject, or
copy one someone published), the app connects to any instance, the
repository holds everything to run one, and all the code is public
(app AGPL-3.0, server SSPL-1.0). Signing in later keeps everything with
GitHub or an email address, now that both are ways in.

The F-Droid listing takes the snap's description again, as its header
says; it had kept the text from before v0.17.0. AGENTS.md no longer
says it declares TetheredNet, which it dropped once the app could
choose its instance.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

snap/snapcraft.yaml

@@ -15,9 +15,9 @@ adopt-info: halkyon-learn # the version, from the app's pubspec.yaml
 summary: Learn with flashcards, studied offline and synced between your devices
 description: |
   Halkyon Learn is a spaced-repetition app for your own material: studysets of
-  flashcards, notes, learning paths and PDFs, reviewed on an FSRS schedule. It
-  works offline and syncs between your devices through an instance anyone can
-  run.
+  flashcards, notes, learning paths and PDFs, reviewed on an FSRS schedule. Every
+  studyset is made by its learners: build yours for any subject, or copy one that
+  someone published. It works offline and syncs between your devices.
 
   Learn from what you watch: play films and shows from your Jellyfin server,
   and every subtitle line is split into dictionary words, with readings above
@@ -29,8 +29,11 @@ description: |
   studysets published to Explore and copied, later releases coming in as
   merges that keep your own edits.
 
-  It starts with an anonymous account; signing in with GitHub later keeps
-  everything. It connects to learn.halkyon.eu, the project's instance.
+  It starts with an anonymous account; signing in with GitHub or an email
+  address later keeps everything. It connects to learn.halkyon.eu, the project's
+  instance, or to any other: everything an instance needs is in the repository,
+  to run your own. The app is AGPL-3.0 and the server SSPL-1.0, so all of its
+  code is public.
 
   Questions and reports: support@halkyon.app
 license: AGPL-3.0-or-later