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>

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