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.

docs/UAT_SMOKE_TESTS.md

@@ -10,6 +10,8 @@ It checks:
 - registration, login, refresh, authenticated profile, dashboard, ratings,
   review queue, and randomized question retrieval;
 - random CCS attempt creation and abandonment;
+- flashcard deck/card creation, studying and grading a card, confirming it
+  drops out of the due queue, and a CSV export/import round trip;
 - public web/legal assets, completion of the Godot loader, and a visible,
   non-uniform canvas without browser or critical asset errors.