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/lib/l10n/app_en.arb

@@ -2870,5 +2870,81 @@
   "studysetViewOnly": "You can view and study this, not change it.",
   "@studysetViewOnly": {"description": "Line on a shared studyset where the person is a Viewer"},
   "studysetViewOnlyRefused": "You can only view this studyset.",
-  "@studysetViewOnlyRefused": {"description": "A write in a studyset shared with the person as a Viewer was refused"}
+  "@studysetViewOnlyRefused": {"description": "A write in a studyset shared with the person as a Viewer was refused"},
+  "shareInviteByHandle": "Handle",
+  "@shareInviteByHandle": {"description": "Inviting someone by their @handle, a choice in the invite dialog"},
+  "shareInviteByEmail": "Email",
+  "@shareInviteByEmail": {"description": "Inviting someone by email address, a choice in the invite dialog"},
+  "shareInviteAddress": "Email address",
+  "@shareInviteAddress": {"description": "Label of the invite dialog's address field"},
+  "shareInviteEmailHint": "They get a link by mail. Only whoever reads that mailbox can join, and the instance keeps the address only as a fingerprint.",
+  "@shareInviteEmailHint": {"description": "Below the address field of the invite dialog"},
+  "shareInviteMailed": "Invitation mailed to {address}",
+  "@shareInviteMailed": {"description": "Snack bar once an invitation by email went out", "placeholders": {"address": {"type": "String"}}},
+  "shareInvitedByEmail": "Invited by email",
+  "@shareInvitedByEmail": {"description": "A member row's name for an invitation mailed to an address, which members never see"},
+  "shareLinkTitle": "Join by link",
+  "@shareLinkTitle": {"description": "Switch on the Share sheet letting people join by the studyset's link"},
+  "shareLinkHint": "Anyone with the link can join as a Viewer. Switching it off ends the link for good.",
+  "@shareLinkHint": {"description": "Below the switch for joining by link"},
+  "shareLinkCopy": "Copy link",
+  "@shareLinkCopy": {"description": "Button copying the studyset's join link"},
+  "shareLinkCopied": "Link copied",
+  "@shareLinkCopied": {"description": "Snack bar once the join link is copied"},
+  "shareRefusedNotAnAddress": "That isn't an email address.",
+  "@shareRefusedNotAnAddress": {"description": "An invitation or code refused for an address that isn't one"},
+  "shareRefusedTooManyMails": "Too many mails to that address for now. Try again in an hour.",
+  "@shareRefusedTooManyMails": {"description": "An invitation or code refused as too many went out this hour"},
+  "shareRefusedMailFailed": "The mail couldn't be sent. Try again later.",
+  "@shareRefusedMailFailed": {"description": "An invitation or code whose mail failed"},
+  "shareRefusedHandleNeeded": "An invitation by mail names you by your handle: choose one in your profile first.",
+  "@shareRefusedHandleNeeded": {"description": "An invitation by email refused as the person has no handle"},
+  "libraryJoin": "Join with a link",
+  "@libraryJoin": {"description": "The Library's menu entry joining a studyset shared by link or invitation"},
+  "joinLinkLabel": "Link or code",
+  "@joinLinkLabel": {"description": "Field of the Join sheet taking a pasted link"},
+  "joinContinue": "Continue",
+  "@joinContinue": {"description": "Button looking up a pasted join link"},
+  "joinInvites": "{inviter} wants to share a studyset with you",
+  "@joinInvites": {"description": "What a join link offers", "placeholders": {"inviter": {"type": "String"}}},
+  "joinSomeone": "Someone",
+  "@joinSomeone": {"description": "Who invites, where they have no public profile"},
+  "joinAsViewer": "You can view and study it.",
+  "@joinAsViewer": {"description": "What joining as a Viewer gives"},
+  "joinAsEditor": "You can view, study and change it.",
+  "@joinAsEditor": {"description": "What joining as an Editor gives"},
+  "joinEmailHint": "This invitation went to an email address. Type it, and you'll get a code there that shows it's yours.",
+  "@joinEmailHint": {"description": "Above the address field of an invitation by email"},
+  "joinSendCode": "Send code",
+  "@joinSendCode": {"description": "Button mailing a code to accept an invitation"},
+  "joinCodeSent": "A code is on its way to {address}.",
+  "@joinCodeSent": {"description": "Once a code to join was mailed", "placeholders": {"address": {"type": "String"}}},
+  "joinCodeLabel": "Code from the mail",
+  "@joinCodeLabel": {"description": "Field taking the mailed code"},
+  "joinAction": "Join",
+  "@joinAction": {"description": "Button joining the shared studyset"},
+  "joinDone": "You joined. The studyset arrives in your Library with the next sync.",
+  "@joinDone": {"description": "Once joined"},
+  "joinAlreadyMember": "You're a member already.",
+  "@joinAlreadyMember": {"description": "A join link to a studyset the person is in already"},
+  "joinNotFound": "This link holds no more: it was switched off, has lapsed or was used already.",
+  "@joinNotFound": {"description": "A join link that holds no more"},
+  "joinWrongCode": "That code isn't right.",
+  "@joinWrongCode": {"description": "A mailed code typed wrong"},
+  "joinCodeExpired": "This code is no longer valid. Send a new one.",
+  "@joinCodeExpired": {"description": "A mailed code used, tried too often or too old"},
+  "joinOtherAddress": "This invitation went to another address.",
+  "@joinOtherAddress": {"description": "A code proving another address than the one invited"},
+  "inboxShared": "Shared with you",
+  "@inboxShared": {"description": "Heading of the Inbox's studysets shared with the person"},
+  "inboxSharedBy": "@{handle} shared {title} with you",
+  "@inboxSharedBy": {"description": "An Inbox entry of a studyset shared with the person", "placeholders": {"handle": {"type": "String"}, "title": {"type": "String"}}},
+  "inboxSharedNoHandle": "{title} was shared with you",
+  "@inboxSharedNoHandle": {"description": "An Inbox entry of a studyset shared by someone without a public profile", "placeholders": {"title": {"type": "String"}}},
+  "inboxSharedArriving": "A studyset on its way",
+  "@inboxSharedArriving": {"description": "The title of a studyset shared with the person that hasn't synced yet"},
+  "inboxSharedOpen": "Open",
+  "@inboxSharedOpen": {"description": "Button opening a studyset shared with the person"},
+  "studysetNew": "New",
+  "@studysetNew": {"description": "Mark on a library card of a studyset someone shared with the person, until opened"}
 }