feat(explore): the Explore tab, and adding a published studyset as a copy

Explore lists what is published on the instance as the old app's feed:
search over titles and authors, filters for languages and kinds of
content, the newest release first, the next page only when asked for;
each studyset opens a summary with all of it. Anonymous people explore
too.

Add to library makes a copy in the app, as an import: through the
person's own repositories, the files among their own files, the tags by
name among their own tags, everything read first, so a release in a
schema this app doesn't know is refused whole. The server never writes
into a learner's space. Every item of a copy names where it came from
(publication, item, release: card schema 9, note 3, step 3, path 2,
PDF 2, studyset 5), merged per side like the rest, for later releases to
be taken from there.

The Publish sheet takes a studyset off Explore again, after asking.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/feature/card/data/card_payload_test.dart

@@ -240,7 +240,7 @@ void main() {
       entryVersion: '0123456789abcdef',
     );
 
-    expect(cardSchemaVersion, 8);
+    expect(cardSchemaVersion, greaterThanOrEqualTo(8));
     expect(decodeCardPayload(encodeCardPayload(content)), content);
     expect(
       decodeCardPayload(