feat(basic-host): Add theme toggle and MCP style variables (#336)

* feat(basic-host): add theme toggle and MCP style variables

Add theme support to basic-host example:
- theme.ts: Global theme manager with light/dark toggle
- host-styles.ts: Full set of MCP CSS variables using light-dark()
- ThemeToggle component with sun/moon icons
- Theme-aware CSS using CSS custom properties
- Pass theme + styles to apps via hostContext
- Notify apps when theme changes via sendHostContextChange

Apps now receive:
- hostContext.theme: 'light' | 'dark'
- hostContext.styles.variables: All 60+ MCP CSS variables
- onhostcontextchanged notifications when theme changes

* test: update e2e goldens for theme changes

examples/basic-host/sandbox.html

@@ -12,6 +12,8 @@
         margin: 0;
         height: 100vh;
         width: 100vw;
+        /* Transparent background allows parent page to show through */
+        background-color: transparent;
       }
       body {
         display: flex;
@@ -26,6 +28,8 @@
         padding: 0px;
         overflow: hidden;
         flex-grow: 1;
+        /* Inherit color scheme from parent for consistent transparency */
+        color-scheme: inherit;
       }
     </style>
   </head>