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_navigation_test.gd

@@ -0,0 +1,23 @@
+extends SceneTree
+
+const NavigationRoutes := preload("res://scripts/ui/navigation_routes.gd")
+
+var failures := 0
+
+
+func _initialize() -> void:
+	_assert(NavigationRoutes.key_for_screen("flashcards_hub") == "flashcards",
+		"the flashcards library screen should activate the Flashcards nav entry")
+	_assert(NavigationRoutes.key_for_screen("flashcards_study") == "flashcards",
+		"the flashcards study screen should activate the Flashcards nav entry")
+	_assert(NavigationRoutes.key_for_screen("dashboard") == "home",
+		"unrelated routes should be unaffected by the new flashcards prefix rule")
+
+	quit(1 if failures > 0 else 0)
+
+
+func _assert(condition: bool, message: String) -> void:
+	if condition:
+		return
+	failures += 1
+	push_error(message)