[Bug] Reviewer keyboard shortcuts stop working after card transitions

When reviewing cards with Incremento enabled, standard Anki keyboard shortcuts (e.g., `Space` to reveal/answer, `1–4` to rate, `e` to edit and so on) stop working starting on the second card of a session.

**Steps to Reproduce**

1. Open a deck and begin a study session with Incremento active.
2. Answer the first card using standard keyboard shortcuts (`Space` / `1`–`4`).
3. Upon loading the second card, attempt to use any standard keyboard shortcut (`Space`, `1`–`4`, `e`, etc.).
4. Observe that the keypress produces no action.
5. Click anywhere on the study canvas with the mouse, then press the shortcut again (it will now work until the next card transition).

**Expected Behavior**
Anki's main reviewer window should retain or reclaim keyboard focus automatically across card transitions so shortcuts function continuously without mouse interaction.

**Environment**
OS: Windows 11
Anki Version: 26.0.5

Awesome plugin. Thanks.

Start at Anki's main reviewer window and trace Incremento's card-transition handling and keyboard-focus behavior. Reproduce the session described in the issue, then verify that Space, 1–4, and e continue working after each transition without clicking the study canvas.

Written by the indexing model from the issue text.