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>

stealth_test.go

@@ -374,3 +374,68 @@ func TestStealthFirefoxScripts_NoChromiumMarkers(t *testing.T) {
 		}
 	}
 }
+
+// --- User-Agent constants ---
+
+func TestDefaultUserAgent_BackwardCompat(t *testing.T) {
+	if DefaultUserAgent != DefaultFirefoxUserAgent {
+		t.Fatal("DefaultUserAgent must equal DefaultFirefoxUserAgent for backward compatibility")
+	}
+}
+
+func TestDefaultFirefoxUserAgent_Content(t *testing.T) {
+	if !strings.Contains(DefaultFirefoxUserAgent, "Firefox") {
+		t.Fatal("DefaultFirefoxUserAgent must contain 'Firefox'")
+	}
+	if strings.Contains(DefaultFirefoxUserAgent, "Chrome") {
+		t.Fatal("DefaultFirefoxUserAgent must not contain 'Chrome'")
+	}
+}
+
+func TestDefaultChromiumUserAgent_Content(t *testing.T) {
+	if !strings.Contains(DefaultChromiumUserAgent, "Chrome") {
+		t.Fatal("DefaultChromiumUserAgent must contain 'Chrome'")
+	}
+	if strings.Contains(DefaultChromiumUserAgent, "Firefox") {
+		t.Fatal("DefaultChromiumUserAgent must not contain 'Firefox'")
+	}
+}
+
+// --- Viewport and UA defaults via mergeOptions ---
+
+func TestMergeOptions_DefaultViewport(t *testing.T) {
+	base := BrowserOptions{
+		Dimensions: Size{Width: 1920, Height: 1080},
+	}
+	got := mergeOptions(base, nil)
+	if got.Dimensions.Width != 1920 || got.Dimensions.Height != 1080 {
+		t.Fatalf("expected default viewport 1920x1080, got %dx%d", got.Dimensions.Width, got.Dimensions.Height)
+	}
+}
+
+func TestMergeOptions_ViewportOverride(t *testing.T) {
+	base := BrowserOptions{
+		Dimensions: Size{Width: 1920, Height: 1080},
+	}
+	got := mergeOptions(base, []BrowserOptions{{Dimensions: Size{Width: 1280, Height: 720}}})
+	if got.Dimensions.Width != 1280 || got.Dimensions.Height != 720 {
+		t.Fatalf("expected overridden viewport 1280x720, got %dx%d", got.Dimensions.Width, got.Dimensions.Height)
+	}
+}
+
+func TestMergeOptions_EmptyUANotOverridden(t *testing.T) {
+	base := BrowserOptions{}
+	got := mergeOptions(base, []BrowserOptions{{Browser: BrowserChromium}})
+	if got.UserAgent != "" {
+		t.Fatalf("expected empty UserAgent after merge with no explicit UA, got %q", got.UserAgent)
+	}
+}
+
+func TestMergeOptions_ExplicitUAPreserved(t *testing.T) {
+	base := BrowserOptions{}
+	customUA := "MyCustomAgent/1.0"
+	got := mergeOptions(base, []BrowserOptions{{UserAgent: customUA}})
+	if got.UserAgent != customUA {
+		t.Fatalf("expected explicit UA %q preserved, got %q", customUA, got.UserAgent)
+	}
+}