Add flashcards feature with spaced repetition scheduling

Adds a full flashcards system: FSRS-based scheduling, cloze/basic
card types, CSV import/export, media storage, and study UI, wired
into navigation and the dashboard alongside existing question flows.

scripts/ui/navigation_routes.gd

@@ -15,6 +15,8 @@ static func key_for_screen(screen_name: String, payload: Variant = null) -> Stri
 		key = "exams"
 	elif screen_name.begins_with("ccs_"):
 		key = "ccs"
+	elif screen_name.begins_with("flashcards_"):
+		key = "flashcards"
 	elif screen_name == "profile":
 		key = "performance"
 	elif screen_name == "settings":