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.

scripts/ui/main.gd

@@ -717,7 +717,7 @@ func _input(event: InputEvent) -> void:
 			_close_account_menu()
 			get_viewport().set_input_as_handled()
 			return
-		if current_screen_name in ["player", "ccs_player"]:
+		if current_screen_name in ["player", "ccs_player", "flashcards_study"]:
 			_toggle_app_menu()
 			get_viewport().set_input_as_handled()
 
@@ -895,9 +895,13 @@ func _toggle_app_menu() -> void:
 		app_menu.visible = true
 		app_menu.size = app_menu.custom_minimum_size
 		app_menu.position = (get_viewport_rect().size - app_menu.size) * 0.5
-		menu_exit_button.visible = (current_screen_name == "player" and BlockSessionService.block_started) or current_screen_name == "ccs_player"
+		menu_exit_button.visible = (current_screen_name == "player" and BlockSessionService.block_started) or current_screen_name in ["ccs_player", "flashcards_study"]
 		if menu_exit_button.visible:
-			menu_exit_button.text = "Leave Case" if current_screen_name == "ccs_player" else "Leave (timer continues)" if BlockSessionService.session_kind == "simulation" else "Exit Run"
+			menu_exit_button.text = (
+				"Leave Case" if current_screen_name == "ccs_player"
+				else "Exit Study Session" if current_screen_name == "flashcards_study"
+				else "Leave (timer continues)" if BlockSessionService.session_kind == "simulation"
+				else "Exit Run")
 			Style.style_chip(menu_exit_button, Style.AMBER)
 			if current_screen_name == "player": BlockSessionService.pause_current_question_timer()
 
@@ -914,6 +918,9 @@ func _exit_run() -> void:
 		var ccs_player = screens.get("ccs_player")
 		if ccs_player != null and ccs_player.has_method("leave_case"): ccs_player.leave_case()
 		return
+	if current_screen_name == "flashcards_study":
+		show_screen("flashcards_hub")
+		return
 	var was_core_run := BlockSessionService.session_kind == "core"
 	var was_review_run := BlockSessionService.session_kind == "review"
 	if BlockSessionService.block_started and BlockSessionService.session_kind != "simulation":