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

internal/examples/session/main.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"github.com/go-via/via"
+	"github.com/go-via/via/h"
+)
+
+func main() {
+	v := via.New()
+
+	v.Page("/", func(c *via.Context) {
+		username := c.Session().GetString("username")
+		flash := c.Session().PopString("flash")
+
+		usernameInput := c.Signal("")
+
+		login := c.Action(func() {
+			name := usernameInput.String()
+			if name != "" {
+				c.Session().Set("username", name)
+				c.Session().Set("flash", "Welcome, "+name+"!")
+				c.Session().RenewToken()
+			}
+			c.Sync()
+		})
+
+		logout := c.Action(func() {
+			c.Session().Set("flash", "Goodbye!")
+			c.Session().Delete("username")
+			c.Sync()
+		})
+
+		c.View(func() h.H {
+			var flashMsg h.H
+			if flash != "" {
+				flashMsg = h.P(h.Text(flash), h.Style("color: green"))
+			}
+
+			if username == "" {
+				return h.Div(
+					flashMsg,
+					h.H1(h.Text("Login")),
+					h.Input(h.Type("text"), h.Placeholder("Username"), usernameInput.Bind()),
+					h.Button(h.Text("Login"), login.OnClick()),
+				)
+			}
+			return h.Div(
+				flashMsg,
+				h.H1(h.Textf("Hello, %s!", username)),
+				h.P(h.Text("Your session persists across page refreshes.")),
+				h.Button(h.Text("Logout"), logout.OnClick()),
+			)
+		})
+	})
+
+	v.Start()
+}