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_en.arb

@@ -15,6 +15,21 @@
 
   "libraryTitle": "Library",
 
+  "studysetCountFlashcards": "{count, plural, =1{1 flashcard} other{{count} flashcards}}",
+  "@studysetCountFlashcards": {"placeholders": {"count": {"type": "int"}}},
+  "studysetNoContent": "No content yet",
+  "studysetActionReview": "Review",
+  "studysetActionStudy": "Study",
+  "studysetActionAdd": "Add content",
+  "studysetArchived": "Archived",
+  "studysetStar": "Star",
+  "studysetUnstar": "Unstar",
+  "studysetOptions": "Options",
+  "studysetMenuEdit": "Edit",
+  "studysetMenuArchive": "Archive",
+  "studysetMenuUnarchive": "Unarchive",
+  "studysetMenuDelete": "Delete",
+
   "exploreTitle": "Explore",
   "exploreEmpty": "Studysets that others share will appear here.",