Distinguish between available and unavailable views on the watch page

[YiranDuan721]

Is your feature request related to a problem? Please describe. Sometimes, for various reasons, only some of the views of a lecture may be available for viewing. Especially in recent times, some lectures do not have a combined view, however, the default selection of combined view on the watch page can be misinterpreted to mean that there is no video available for this lecture. I would like to see a way to differentiate between the available and unavailable views on the watch page.

Describe the solution you'd like

1. Gray out unavailable views in the views selection list, and also disable the corresponding button:

2. When loading the watch page, load the first available view according to the following priorities:
   1. Combined view
   2. Split view
   3. Presentation-only view
   4. Camera-only view