Rebuild the admin dashboard as a Svelte 5 + TypeScript app

Frontend in web/ (Vite via bun, pure CSS, no component libraries),
built to internal/dashboard/dist and embedded into the binary; the
Dockerfile builds it in a bun stage. Old static dashboard.html removed.

internal/server/admin_test.go

@@ -2,11 +2,15 @@ package server
 
 import (
 	"encoding/json"
+	"io"
+	"io/fs"
 	"net/http"
 	"net/http/httptest"
 	"strings"
 	"testing"
 	"time"
+
+	"gitea.tzgyn.dev/TZGyn/tunnel/internal/dashboard"
 )
 
 func TestAdminOverview(t *testing.T) {
@@ -98,19 +102,21 @@ func TestDashboardPage(t *testing.T) {
 	srv := httptest.NewServer(api)
 	defer srv.Close()
 
+	if _, err := fs.Stat(dashboard.FS(), "index.html"); err != nil {
+		t.Skip("dashboard not built (run `bun install && bun run build` in web/)")
+	}
 	for _, path := range []string{"/", "/admin"} {
 		resp, err := http.Get(srv.URL + path)
 		if err != nil {
 			t.Fatal(err)
 		}
-		body := make([]byte, 512)
-		n, _ := resp.Body.Read(body)
+		body, _ := io.ReadAll(resp.Body)
 		resp.Body.Close()
 		if resp.StatusCode != 200 || !strings.Contains(resp.Header.Get("Content-Type"), "text/html") {
 			t.Errorf("%s: got %d %q", path, resp.StatusCode, resp.Header.Get("Content-Type"))
 		}
-		if !strings.Contains(string(body[:n]), "tunneld admin") {
-			t.Errorf("%s: unexpected body", path)
+		if !strings.Contains(string(body), `id="app"`) {
+			t.Errorf("%s: body does not look like the built dashboard", path)
 		}
 	}
 }