feat: add SPA navigation with view transitions

Swap page content over the existing SSE connection without full page
loads. A persistent Context resets its page-specific state (signals,
actions, intervals, subscriptions) on navigate while preserving the
SSE stream, CSRF token, and session.

- c.Navigate(path) for programmatic SPA navigation from actions
- Injected JS intercepts same-origin <a> clicks (opt out with
  data-via-no-boost) and handles popstate for back/forward
- v.Layout() wraps pages in a shared shell for DRY nav/chrome
- View Transition API integration via WithViewTransitions() on
  PatchElements and h.DataViewTransition() helper
- POST /_navigate endpoint with CSRF validation and rate limiting
- pageStopChan cancels page-level OnInterval goroutines on navigate
- Includes SPA example with layout, counter, and live clock pages

internal/examples/spa/main.go

@@ -0,0 +1,91 @@
+package main
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/ryanhamamura/via"
+	. "github.com/ryanhamamura/via/h"
+)
+
+func main() {
+	v := via.New()
+	v.Config(via.Options{
+		DocumentTitle: "SPA Navigation",
+		ServerAddress: ":7331",
+	})
+
+	v.AppendToHead(
+		Raw(`<link rel="preconnect" href="https://fonts.googleapis.com">`),
+		Raw(`<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>`),
+		Raw(`<link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;600;700&display=swap" rel="stylesheet">`),
+		Raw(`<style>body{font-family:'Inter',sans-serif;margin:0;background:#111;color:#eee}</style>`),
+	)
+
+	v.Layout(func(content func() H) H {
+		return Div(
+			Nav(
+				Style("display:flex;gap:1rem;padding:1rem;background:#222;"),
+				A(Href("/"), Text("Home"), Style("color:#fff")),
+				A(Href("/counter"), Text("Counter"), Style("color:#fff")),
+				A(Href("/clock"), Text("Clock"), Style("color:#fff")),
+				A(Href("https://github.com"), Text("GitHub (external)"), Style("color:#888")),
+				A(Href("/"), Text("Full Reload"), Attr("data-via-no-boost"), Style("color:#f88")),
+			),
+			Main(Style("padding:1rem"), content()),
+		)
+	})
+
+	// Home page
+	v.Page("/", func(c *via.Context) {
+		goCounter := c.Action(func() { c.Navigate("/counter", false) })
+
+		c.View(func() H {
+			return Div(
+				H1(Text("Home"), DataViewTransition("page-title")),
+				P(Text("Click the nav links above — no page reload, no white flash.")),
+				P(Text("Or navigate programmatically:")),
+				Button(Text("Go to Counter"), goCounter.OnClick()),
+			)
+		})
+	})
+
+	// Counter page — demonstrates signals and actions survive within a page,
+	// but reset on navigate away and back.
+	v.Page("/counter", func(c *via.Context) {
+		count := 0
+		increment := c.Action(func() { count++; c.Sync() })
+		goHome := c.Action(func() { c.Navigate("/", false) })
+
+		c.View(func() H {
+			return Div(
+				H1(Text("Counter"), DataViewTransition("page-title")),
+				P(Textf("Count: %d", count)),
+				Button(Text("+1"), increment.OnClick()),
+				Button(Text("Go Home"), goHome.OnClick(), Style("margin-left:0.5rem")),
+			)
+		})
+	})
+
+	// Clock page — demonstrates OnInterval cleanup on navigate.
+	v.Page("/clock", func(c *via.Context) {
+		now := time.Now().Format("15:04:05")
+		c.OnInterval(time.Second, func() {
+			now = time.Now().Format("15:04:05")
+			c.Sync()
+		})
+
+		c.View(func() H {
+			return Div(
+				H1(Text("Clock"), DataViewTransition("page-title")),
+				P(Text("This page has an OnInterval that ticks every second.")),
+				P(Textf("Current time: %s", now)),
+				P(Text("Navigate away and back — the old interval stops, a new one starts.")),
+				P(Textf("Proof this is a fresh page init: random = %d", time.Now().UnixNano()%1000)),
+			)
+		})
+	})
+
+	fmt.Println("SPA example running at http://localhost:7331")
+	v.Start()
+}