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/android/app/src/main/AndroidManifest.xml

@@ -53,6 +53,19 @@
                 <action android:name="android.intent.action.MAIN"/>
                 <category android:name="android.intent.category.LAUNCHER"/>
             </intent-filter>
+            <!-- Links to join a shared studyset (Halkyon's ADR 0044, D39), opened by the
+                 app itself (App Links): on learn.halkyon.eu, whose assetlinks.json names
+                 this app's key (deploy/Caddyfile); go_router takes /join/<token>. -->
+            <meta-data android:name="flutter_deeplinking_enabled" android:value="true" />
+            <intent-filter android:autoVerify="true">
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data
+                    android:scheme="https"
+                    android:host="learn.halkyon.eu"
+                    android:pathPrefix="/join/" />
+            </intent-filter>
         </activity>
         <!-- Receives the way back from SimpleLogin's sign-in (app.halkyon.learn://simplelogin-callback). -->
         <activity