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/shadertoy-server/src/mcp-app.css

@@ -69,7 +69,7 @@ html, body {
   align-items: center;
   justify-content: center;
   transition: background 0.2s, opacity 0.2s;
-  opacity: 0; /* Initially invisible, shown on widget hover */
+  opacity: 0; /* Initially invisible, shown on View hover */
   z-index: 100;
 }
 
@@ -91,7 +91,7 @@ html, body {
   display: flex;
 }
 
-/* Show button on widget hover */
+/* Show button on View hover */
 .main:hover .fullscreen-btn.available {
   opacity: 0.7;
 }