enhance: thread-safe CookieJar, SameSite cookie attr, dynamic Google countries

- Wrap staticCookieJar in struct with sync.RWMutex for thread safety
- Add SameSite field to Cookie struct with Strict/Lax/None constants
- Update Playwright cookie conversion functions for SameSite
- Replace hardcoded 4-country switch with dynamic country code generation

Closes #20, #22, #23
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cookiejar.go

@@ -6,6 +6,15 @@ import (
 	"time"
 )
 
+// SameSite represents the SameSite attribute of a cookie.
+type SameSite string
+
+const (
+	SameSiteStrict SameSite = "Strict"
+	SameSiteLax    SameSite = "Lax"
+	SameSiteNone   SameSite = "None"
+)
+
 type Cookie struct {
 	Host     string
 	Path     string
@@ -14,6 +23,7 @@ type Cookie struct {
 	HttpOnly bool
 	Name     string
 	Value    string
+	SameSite SameSite
 }
 
 func (c Cookie) IsTargetMatch(target string) (bool, error) {