feat(app): the library's studyset card

A studyset in the library: a bar in its colour, its title with the secondary
title as a kicker, what it holds, and beside the title the action it
invites inside a ring its mature cards fill, with a badge counting the due
cards (at most "99+", growing inwards so it never passes the card's edge).
The action is a review when reviews are due, studying when only new cards
are, and adding content otherwise, which opens the studyset: the card only
invites a session that has something to study. A long press stars it, as
does the star, which a hovering pointer reveals and which stays while
starred; the options edit, archive or unarchive, and delete it. An archived
studyset rests: dimmed, neutral, paused. Until its progress is known, a
muted bar stands in for "No content yet", which it might not be.

A starred star is amber, dark enough in the light theme to stand out from
the cards as a graphic must (3:1), a colour the theme extras now hold. It is
Material's filled star: Lucide has no filled one, and the fill is what says
the star is on.

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

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -37,6 +37,53 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get libraryTitle => 'Library';
 
+  @override
+  String studysetCountFlashcards(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count flashcards',
+      one: '1 flashcard',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get studysetNoContent => 'No content yet';
+
+  @override
+  String get studysetActionReview => 'Review';
+
+  @override
+  String get studysetActionStudy => 'Study';
+
+  @override
+  String get studysetActionAdd => 'Add content';
+
+  @override
+  String get studysetArchived => 'Archived';
+
+  @override
+  String get studysetStar => 'Star';
+
+  @override
+  String get studysetUnstar => 'Unstar';
+
+  @override
+  String get studysetOptions => 'Options';
+
+  @override
+  String get studysetMenuEdit => 'Edit';
+
+  @override
+  String get studysetMenuArchive => 'Archive';
+
+  @override
+  String get studysetMenuUnarchive => 'Unarchive';
+
+  @override
+  String get studysetMenuDelete => 'Delete';
+
   @override
   String get exploreTitle => 'Explore';