fix(#491): auto-disable viewport for --start-maximized and --window-size args (#492)

Fixes #491

When `--start-maximized` or `--window-size` is passed as a browser arg, Playwright's default viewport (1280x720) overrides the browser's own window sizing, making those flags have no effect on the page content.

This change auto-detects those args and sets `viewport: null` so Playwright defers to the browser's window size. Explicit viewport values still take priority.

Also allows `viewport: null` in the launch protocol for agents that want to disable viewport emulation directly.

src/types.ts

@@ -10,7 +10,7 @@ export interface BaseCommand {
 export interface LaunchCommand extends BaseCommand {
   action: 'launch';
   headless?: boolean;
-  viewport?: { width: number; height: number };
+  viewport?: { width: number; height: number } | null;
   browser?: 'chromium' | 'firefox' | 'webkit';
   headers?: Record<string, string>;
   executablePath?: string;
@@ -850,7 +850,7 @@ export interface TabCloseCommand extends BaseCommand {
 
 export interface WindowNewCommand extends BaseCommand {
   action: 'window_new';
-  viewport?: { width: number; height: number };
+  viewport?: { width: number; height: number } | null;
 }
 
 // Union of all command types