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/src/global.css

@@ -1,10 +1,38 @@
+:root {
+  color-scheme: light dark;
+
+  /* Light theme (default) */
+  --color-bg: #ffffff;
+  --color-bg-secondary: #f5f5f5;
+  --color-text: #1f2937;
+  --color-text-secondary: #6b7280;
+  --color-border: #e5e7eb;
+  --color-primary: #1e3a5f;
+  --color-primary-hover: #2d4a7c;
+}
+
+[data-theme="dark"] {
+  --color-bg: #1a1a1a;
+  --color-bg-secondary: #2d2d2d;
+  --color-text: #f3f4f6;
+  --color-text-secondary: #9ca3af;
+  --color-border: #404040;
+  --color-primary: #3b82f6;
+  --color-primary-hover: #60a5fa;
+}
+
 * {
   box-sizing: border-box;
 }
 
 html, body {
+  margin: 0;
+  padding: 0;
   font-family: system-ui, -apple-system, sans-serif;
   font-size: 1rem;
+  background-color: var(--color-bg);
+  color: var(--color-text);
+  transition: background-color 0.2s, color 0.2s;
 }
 
 code {