fix: remove "system" from theme options (#40)

The host should resolve "system" to either "light" or "dark" before
passing the theme to the app. Apps only need to know the actual
resolved theme value.

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

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

specification/draft/apps.mdx

@@ -399,7 +399,7 @@ interface HostContext {
     tool: Tool,
   };
   /** Current color theme preference */
-  theme?: "light" | "dark" | "system";
+  theme?: "light" | "dark";
   /** How the UI is currently displayed */
   displayMode?: "inline" | "fullscreen" | "pip";
   /** Display modes the host supports */

src/types.ts

@@ -501,7 +501,7 @@ export const McpUiHostContextSchema: z.ZodType<McpUiHostContext> = z.object({
       tool: ToolSchema,
     })
     .optional(),
-  theme: z.enum(["light", "dark", "system"]).optional(),
+  theme: z.enum(["light", "dark"]).optional(),
   displayMode: z.enum(["inline", "fullscreen", "pip"]).optional(),
   availableDisplayModes: z.array(z.string()).optional(),
   viewport: z