ensure multiple answer buttons don't get accepted when mashed in v3

We want to preserve the backend checks, because they may also detect
bugs. So we do the debouncing on the frontend instead, transitioning
into a temporary state until the background op completes.

https://forums.ankiweb.net/t/anki-2-1-50-beta/15608/74

qt/aqt/reviewer.py

@@ -121,7 +121,7 @@ class Reviewer:
         self._answeredIds: list[CardId] = []
         self._recordedAudio: str | None = None
         self.typeCorrect: str = None  # web init happens before this is set
-        self.state: str | None = None
+        self.state: Literal["question", "answer", "transition", None] = None
         self._refresh_needed: RefreshNeeded | None = None
         self._v3: V3CardInfo | None = None
         self._state_mutation_key = str(random.randint(0, 2 ** 64 - 1))
@@ -420,6 +420,7 @@ class Reviewer:
                 if sched.state_is_leech(answer.new_state):
                     self.onLeech()
 
+            self.state = "transition"
             answer_card(parent=self.mw, answer=answer).success(
                 after_answer
             ).run_in_background(initiator=self)