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.

tests/flashcard_cloze_test.gd

@@ -0,0 +1,32 @@
+extends SceneTree
+
+const Cloze := preload("res://scripts/ui/flashcard_cloze.gd")
+
+var failures := 0
+
+
+func _initialize() -> void:
+	var text := "The capital of {{c1::France}} is {{c2::Paris::city}}."
+
+	_assert(Cloze.render_front(text, 1) == "The capital of [...] is Paris.",
+		"front should hide the target cloze number and reveal the rest")
+	_assert(Cloze.render_front(text, 2) == "The capital of France is [city].",
+		"front should show the hint in brackets when one is provided")
+	_assert(Cloze.render_back(text) == "The capital of France is Paris.",
+		"back should reveal every cloze answer regardless of variant")
+
+	var multiline := "Line one\n{{c1::multi\nline\nanswer}}\nLine two"
+	_assert(Cloze.render_back(multiline) == "Line one\nmulti\nline\nanswer\nLine two",
+		"cloze content spanning multiple lines should still parse as one deletion")
+
+	_assert(Cloze.render_front("no markers here", 1) == "no markers here",
+		"text without cloze markers should render unchanged")
+
+	quit(1 if failures > 0 else 0)
+
+
+func _assert(condition: bool, message: String) -> void:
+	if condition:
+		return
+	failures += 1
+	push_error(message)