fix: remove 'system' from theme type (#79)

Remove 'system' option from theme, leaving only 'light' | 'dark'.

This aligns the SDK with:
- The spec (which already only has light/dark)
- OpenAI Apps SDK compatibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

src/types.ts

@@ -425,7 +425,7 @@ export interface McpUiHostContext {
    * Current color theme preference.
    * @example "dark"
    */
-  theme?: "light" | "dark" | "system";
+  theme?: "light" | "dark";
   /**
    * How the UI is currently displayed.
    * @example "inline"