fix: set default viewport for NewBrowser and align User-Agent with engine

NewBrowser previously had no viewport (strong headless signal) and used a
Firefox User-Agent unconditionally, even for Chromium instances (detectable
mismatch).

Add per-engine UA constants (DefaultFirefoxUserAgent, DefaultChromiumUserAgent)
and auto-select the matching UA in initBrowser when the caller hasn't set one
explicitly. Keep DefaultUserAgent as a backward-compatible alias.

Add 1920x1080 default viewport to NewBrowser (most common desktop resolution).
NewInteractiveBrowser keeps its existing 1280x720 default but also gains
engine-aware UA selection.

Closes #70

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

interactive.go

@@ -59,10 +59,9 @@ type interactiveBrowser struct {
 func NewInteractiveBrowser(ctx context.Context, opts ...BrowserOptions) (InteractiveBrowser, error) {
 	var thirtySeconds = 30 * time.Second
 	opt := mergeOptions(BrowserOptions{
-		UserAgent: DefaultUserAgent,
-		Browser:   BrowserFirefox,
-		Timeout:   &thirtySeconds,
-		Stealth:   Bool(true),
+		Browser: BrowserFirefox,
+		Timeout: &thirtySeconds,
+		Stealth: Bool(true),
 		Dimensions: Size{
 			Width:  1280,
 			Height: 720,