Rebuild flashcard study screen from mockup, with real backend-backed stats

Study screen now matches the provided mockup's layout: a "Current
Deck" panel with a circular mastery-progress ring, and a "Spaced
Repetition" panel (New/Due/Learning/Review counts, streak, daily
goal) on either side of the card. Kept the app's real global sidebar
rather than the mockup's own nav, per direction.

Mastery/Streak/Daily Goal weren't tracked anywhere, so this adds real
backend support rather than showing fabricated numbers:
- Mastered = graduated to review state AND FSRS stability >= 21 days
  (FlashcardScheduleStore.MasteryStabilityDaysThreshold), stricter
  than the plain Review-state count
- CurrentStreakDays = consecutive UTC days with a review, computed
  from review-log timestamps in the new pure FlashcardScheduling
  .ComputeStreakDays (unit tested, habit-streak semantics: doesn't
  reset to 0 the instant a new day starts before today's first review)
- DailyGoal/ReviewsToday reuse the account's existing ReviewsPerDay
  setting rather than inventing a new one
Also removes the exit "X" button in favor of the same Escape/pause
menu the question-player flow uses (main.gd's shared app_menu, with
an "Exit Study Session" action), and adds a new circular_progress.gd
reusable ring control plus a real "Options" menu (bury/suspend the
current card) wired to schedule endpoints that had no UI before.

api/QuestionsApi/Models/FlashcardModels.cs

@@ -175,6 +175,9 @@ public sealed class FlashcardDeckSummary
     public int Due { get; set; }
     public int New { get; set; }
     public int Suspended { get; set; }
+    public int Learning { get; set; }
+    public int Review { get; set; }
+    public int Mastered { get; set; }
 }
 
 public sealed class FlashcardLibraryResponse
@@ -266,6 +269,11 @@ public sealed class FlashcardDashboardCounts
     public int Due { get; set; }
     public int NewAvailable { get; set; }
     public int ReviewedThisWeek { get; set; }
+    public int Learning { get; set; }
+    public int Review { get; set; }
+    public int CurrentStreakDays { get; set; }
+    public int ReviewsToday { get; set; }
+    public int DailyGoal { get; set; }
 }
 
 public static class FlashcardImportFormats