feature: add stealth mode, launch args, and init scripts to BrowserOptions

Add anti-bot-detection evasion support to reduce blocking by sites like
archive.ph. Stealth mode is enabled by default for all browsers and applies
common evasions: navigator.webdriver override, plugin/mimeType spoofing,
window.chrome stub, and outerWidth/outerHeight fixes. For Chromium,
--disable-blink-features=AutomationControlled is also added.

New BrowserOptions fields:
- Stealth *bool: toggle stealth presets (default true)
- LaunchArgs []string: custom browser launch arguments
- InitScripts []string: JavaScript injected before page scripts

Closes #56

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

browser_init.go

@@ -52,6 +52,21 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 		return nil, ErrInvalidBrowserSelection
 	}
 
+	// Collect launch args and init scripts, starting with any stealth-mode presets.
+	stealth := opt.Stealth == nil || *opt.Stealth
+	var launchArgs []string
+	var initScripts []string
+
+	if stealth {
+		if opt.Browser == BrowserChromium {
+			launchArgs = append(launchArgs, stealthChromiumArgs...)
+		}
+		initScripts = append(initScripts, stealthInitScripts...)
+	}
+
+	launchArgs = append(launchArgs, opt.LaunchArgs...)
+	initScripts = append(initScripts, opt.InitScripts...)
+
 	var browser playwright.Browser
 	launch := true
 
@@ -71,9 +86,13 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 
 	if launch {
 		headless := opt.ShowBrowser == nil || !*opt.ShowBrowser
-		browser, err = bt.Launch(playwright.BrowserTypeLaunchOptions{
+		launchOpts := playwright.BrowserTypeLaunchOptions{
 			Headless: playwright.Bool(headless),
-		})
+		}
+		if len(launchArgs) > 0 {
+			launchOpts.Args = launchArgs
+		}
+		browser, err = bt.Launch(launchOpts)
 		if err != nil {
 			return nil, fmt.Errorf("failed to launch browser: %w", err)
 		}
@@ -103,6 +122,12 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 		return nil, fmt.Errorf("failed to create browser context: %w", err)
 	}
 
+	for _, script := range initScripts {
+		if err := bctx.AddInitScript(playwright.Script{Content: &script}); err != nil {
+			return nil, fmt.Errorf("failed to add init script: %w", err)
+		}
+	}
+
 	if opt.CookieJar != nil {
 		cookies, err := opt.CookieJar.GetAll()
 		if err != nil {
@@ -158,6 +183,15 @@ func mergeOptions(base BrowserOptions, opts []BrowserOptions) BrowserOptions {
 		if o.ShowBrowser != nil {
 			base.ShowBrowser = o.ShowBrowser
 		}
+		if len(o.LaunchArgs) > 0 {
+			base.LaunchArgs = append(base.LaunchArgs, o.LaunchArgs...)
+		}
+		if len(o.InitScripts) > 0 {
+			base.InitScripts = append(base.InitScripts, o.InitScripts...)
+		}
+		if o.Stealth != nil {
+			base.Stealth = o.Stealth
+		}
 	}
 	return base
 }