SEP: Align on UI terminology

[idosal]

The spec has multiple names for the UI -

1. Guest UI refers to the rendered resources.
2. Widget refers to the rendered resources.
3. App is usually used for the concept or overall application (an App as an MCP server may be comprised of multiple widgets and tools). However, in the code, AppRenderer and appBridge refer to a single rendered resource.

We need to align on the terminology.

[glen-84]

FWIW, I prefer the term Component.

If the AppRenderer is rendering a single component, then perhaps it should be named ComponentRenderer? (and likewise, componentBridge).

[nickcoai]

I'm a fan of Component too - I don't really like "Widget" as I feel these things could be quite large and complex, not just something on the side (widgets feel "small")