Add counter example demonstrating signals

Basic example showing:
- Creating a signal with NewSignal
- Reading value with Get()
- Writing value with Set()
- Button click handler
- Reactive text that updates when signal changes

Closes #4

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/counter/main.go

@@ -0,0 +1,31 @@
+//go:build js && wasm
+
+package main
+
+import (
+	"fmt"
+
+	"git.flowmade.one/flowmade-one/iris/reactive"
+	"git.flowmade.one/flowmade-one/iris/ui"
+)
+
+func main() {
+	// Create a signal with initial value 0
+	count := reactive.NewSignal(0)
+
+	view := ui.NewView()
+
+	// Reactive text that updates when count changes
+	view.Child(ui.TextFromFunction(func() string {
+		return fmt.Sprintf("Count: %d", count.Get())
+	}))
+
+	// Button that increments the count on click
+	view.Child(ui.Button(func() {
+		count.Set(count.Get() + 1)
+	}, ui.TextFromString("Increment")))
+
+	ui.NewApp(view)
+
+	select {}
+}