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/Services/Flashcards/FlashcardScheduling.cs

@@ -11,4 +11,25 @@ public static class FlashcardScheduling
     // already been consumed on the client's current local study day.
     public static int ResolveDailyBudget(int globalLimit, int? deckOverride, int consumedToday) =>
         Math.Max(0, (deckOverride ?? globalLimit) - consumedToday);
+
+    // A streak counts consecutive calendar days with at least one review, ending either today or
+    // yesterday (so the streak doesn't visibly reset to 0 the moment a new day starts before
+    // today's first review) - the same semantics as most habit-streak trackers.
+    public static int ComputeStreakDays(IReadOnlyCollection<DateTime> reviewTimestamps, DateTime todayUtc)
+    {
+        var reviewDays = reviewTimestamps.Select(v => v.Date).ToHashSet();
+        var cursor = todayUtc.Date;
+        if (!reviewDays.Contains(cursor))
+        {
+            cursor = cursor.AddDays(-1);
+            if (!reviewDays.Contains(cursor)) return 0;
+        }
+        var streak = 0;
+        while (reviewDays.Contains(cursor))
+        {
+            streak++;
+            cursor = cursor.AddDays(-1);
+        }
+        return streak;
+    }
 }