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/flashcard_cloze.gd

@@ -0,0 +1,41 @@
+extends RefCounted
+
+# Mirrors api/QuestionsApi/Services/Flashcards/FlashcardClozeParser.cs so the study screen can
+# render a cloze note's front/back without a round trip to the server.
+
+static func _regex() -> RegEx:
+	var regex := RegEx.new()
+	regex.compile("(?s)\\{\\{c(\\d+)::((?:(?!\\}\\}).)*)\\}\\}")
+	return regex
+
+
+static func render_front(text: String, cloze_number: int) -> String:
+	var regex := _regex()
+	var result := ""
+	var last_end := 0
+	for match in regex.search_all(text):
+		result += text.substr(last_end, match.get_start() - last_end)
+		var number := int(match.get_string(1))
+		var inner := match.get_string(2)
+		var parts := inner.split("::", true, 1)
+		if number == cloze_number:
+			var hint := parts[1].strip_edges() if parts.size() > 1 else ""
+			result += ("[%s]" % hint) if not hint.is_empty() else "[...]"
+		else:
+			result += parts[0].strip_edges()
+		last_end = match.get_end()
+	result += text.substr(last_end)
+	return result
+
+
+static func render_back(text: String) -> String:
+	var regex := _regex()
+	var result := ""
+	var last_end := 0
+	for match in regex.search_all(text):
+		result += text.substr(last_end, match.get_start() - last_end)
+		var inner := match.get_string(2)
+		result += inner.split("::", true, 1)[0].strip_edges()
+		last_end = match.get_end()
+	result += text.substr(last_end)
+	return result