Split reviewer page state

[andrewsanchez]

Source split from ankitects#4289.

Breakdown sections covered:

• 2. New Svelte reviewer page scaffolding
• 4. Parent/iframe request handling from the outer page
• 5. NextCardData frontend consumption
• 7. ReviewerAction frontend consumption
• 8. PlayAVTags frontend consumption
• 13. Edit button
• 24. Image pre-loading

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.
• The top of this stack was verified with an empty git diff HEAD origin/pr/4289.

Follow-up notes from the breakdown:

• This layer intentionally keeps the original state-machine implementation unchanged.
• Remaining open design items are documented in the earlier PR bodies instead of being fixed here.