feat(app,learn): invite to a shared studyset by email or link, and join

The apps' half of ADR 0044's step 5 (D37 to D40), on the instance's
half of the commit before.

halkyon_app's SharedSpaceGateway asks for inviting by mail, the
preview of a link, the code proving an address, accepting a mailed
invitation, a space's link (read, switched, joined) and marking a
share seen; SpaceInfo says whether a share is new and who made it,
a member whether they were invited by mail. joinTokenOf reads a link
or its token as pasted. The fake keeps the mails and codes it would
send, for the apps' tests.

In Learn:
- the Owner's Share sheet invites by handle or by email, lists an
  invitation mailed by its role alone, and switches the space's link,
  copying it while on;
- the Library's menu has Join with a link, and a link opened on
  Android (/join/<token> on learn.halkyon.eu, verified through the
  instance's assetlinks) opens the same on a screen of its own: who
  invites and as whom, then Join, or for a mailed invitation the
  address and the code mailed there;
- a studyset someone shared is new until opened: in the Inbox
  ("@alice shared Japanese with you", Open or Leave), on the Profile
  tab's badge and as New on its library card (NewSharesCubit, read
  again after every merge).

Whats-New: Invite people to a shared studyset by email or with a link, join one from the Library's Join with a link, and see in the Inbox what was shared with you.
Whats-New-De: Lade Leute per E-Mail oder Link zu einem geteilten Lernset ein, tritt einem über „Mit einem Link beitreten“ in der Bibliothek bei und sieh im Posteingang, was mit dir geteilt wurde.

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/test/app/long_names_test.dart

@@ -128,6 +128,7 @@ void main() {
       String location, {
       AuthGateway? authGateway,
       SignInAddresses? signInAddresses,
+      SharedSpaceGateway? sharedSpaces,
     }) async {
       tester.view.physicalSize = Size(width, 800);
       tester.view.devicePixelRatio = 1;
@@ -144,6 +145,7 @@ void main() {
           tagGateway: tagGateway,
           settingsGateway: settingsGateway,
           discussionGateway: discussions,
+          sharedSpaces: sharedSpaces,
         ),
         location: location,
       );
@@ -231,6 +233,27 @@ void main() {
 
       expect(find.textContaining('anna anna'), findsOneWidget);
     });
+
+    testWidgets('on $window, the Inbox with a long studyset someone with a '
+        'long handle shared', (tester) async {
+      final handle = long('owner', 30).replaceAll(' ', '_');
+      final spaces = FakeSharedSpaceServer(FakeSharedKeysServer())
+        ..addPerson('owner', handle: handle)
+        ..addPerson('person', handle: 'person');
+      final space = await spaces.as('owner').create('studyset');
+      await spaces.as('owner').invite(space, 'person', SpaceRole.viewer);
+      studysets.shared['japanese'] = (space: space, viewOnly: true);
+      await show(
+        tester,
+        RoutePaths.profile,
+        authGateway: FakeAuthGateway(
+          status: const SignedIn(authUserId: 'person', isIdentified: true),
+        ),
+        sharedSpaces: spaces.as('person'),
+      );
+
+      expect(find.textContaining('@$handle shared'), findsOneWidget);
+    });
   }
 }