docs: standardize UI terminology to use "view" consistently (#325)

* docs: standardize UI terminology to use "view" consistently

Replaces "widget" and "Guest UI" terminology with "view" across
the codebase as decided in the community survey.

This change updates:
- Specification document
- SDK source code and JSDoc comments
- Example servers and documentation
- Plugin documentation and skills

Closes #71

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/pdf-server/server.ts

@@ -189,7 +189,7 @@ The viewer supports zoom, navigation, text selection, and fullscreen mode.`,
         ],
         structuredContent: result,
         _meta: {
-          widgetUUID: randomUUID(),
+          viewUUID: randomUUID(),
         },
       };
     },