feat: add cookie-based session support using alexedwards/scs (#1)

- Add Session wrapper with typed getters (GetString, GetInt, GetBool, etc.)
- Add flash message support via Pop methods (PopString, PopInt, etc.)
- Add session utilities: Exists, Keys, ID, Clear, Destroy, RenewToken
- Create default session manager in New() for zero-config usage
- Allow custom session manager via Options.SessionManager
- Wrap mux with scs LoadAndSave middleware in Start()
- Add session example demonstrating login/logout with flash messages

configuration.go

@@ -1,5 +1,7 @@
 package via
 
+import "github.com/alexedwards/scs/v2"
+
 type LogLevel int
 
 const (
@@ -30,4 +32,9 @@ type Options struct {
 
 	// Plugins to extend the capabilities of the `Via` application.
 	Plugins []Plugin
+
+	// SessionManager enables cookie-based sessions. If set, Via wraps handlers
+	// with scs LoadAndSave middleware. Configure the session manager before
+	// passing it (lifetime, cookie settings, store, etc).
+	SessionManager *scs.SessionManager
 }