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.Tests/FlashcardSchedulingTests.cs

@@ -38,4 +38,68 @@ public sealed class FlashcardSchedulingTests
     {
         Assert.Equal(expected, FlashcardScheduling.ResolveDailyBudget(globalLimit, deckOverride, consumedToday));
     }
+
+    private static readonly DateTime Today = new(2026, 3, 10, 0, 0, 0, DateTimeKind.Utc);
+
+    [Fact]
+    public void ComputeStreakDays_NoReviews_IsZero()
+    {
+        Assert.Equal(0, FlashcardScheduling.ComputeStreakDays([], Today));
+    }
+
+    [Fact]
+    public void ComputeStreakDays_ReviewedTodayOnly_IsOne()
+    {
+        var timestamps = new List<DateTime> { Today.AddHours(9) };
+
+        Assert.Equal(1, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
+    }
+
+    [Fact]
+    public void ComputeStreakDays_ThreeConsecutiveDaysEndingToday_IsThree()
+    {
+        var timestamps = new List<DateTime>
+        {
+            Today.AddDays(-2).AddHours(8), Today.AddDays(-1).AddHours(20), Today.AddHours(7)
+        };
+
+        Assert.Equal(3, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
+    }
+
+    [Fact]
+    public void ComputeStreakDays_NotYetReviewedToday_StillCountsThroughYesterday()
+    {
+        // The streak shouldn't visibly reset to 0 the instant a new calendar day begins, before
+        // the user has had a chance to study today.
+        var timestamps = new List<DateTime> { Today.AddDays(-2).AddHours(8), Today.AddDays(-1).AddHours(20) };
+
+        Assert.Equal(2, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
+    }
+
+    [Fact]
+    public void ComputeStreakDays_GapBeforeYesterday_BreaksTheStreak()
+    {
+        var timestamps = new List<DateTime>
+        {
+            Today.AddDays(-10).AddHours(8), Today.AddDays(-1).AddHours(20), Today.AddHours(7)
+        };
+
+        Assert.Equal(2, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
+    }
+
+    [Fact]
+    public void ComputeStreakDays_MissedYesterdayAndToday_IsZero()
+    {
+        var timestamps = new List<DateTime> { Today.AddDays(-3).AddHours(8) };
+
+        Assert.Equal(0, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
+    }
+
+    [Fact]
+    public void ComputeStreakDays_MultipleReviewsSameDay_CountOnceTowardTheStreak()
+    {
+        var timestamps = new List<DateTime> { Today.AddHours(7), Today.AddHours(8), Today.AddHours(9) };
+
+        Assert.Equal(1, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
+    }
 }