Split reviewer toggle and API access

[andrewsanchez]

Source split from ankitects#4289.

Breakdown sections covered:

• 1. Preference toggle: new_reviewer opt-in
• 35. Main-window API access

Source fidelity:

• This PR copies exact final file contents from PR Feat/Svelte port for reviewer ankitects/anki#4289.
• No implementation changes were made while splitting.
• Commit author is preserved as Luc Mcgrady.

Follow-up notes from the breakdown:

• The reviewer toggle requires restart because API access is configured when the webview is initialized.
• API access should ideally be scoped to the reviewer webview rather than the whole main window.
• The toggle may be better stored in profile metadata instead of collection config.