Serve the index page for an empty URL path instead of panicking (closes #149)

setupSPA took the file name from request.URL.Path[1:], which panics
when the path is empty, as it is for an absolute-form request line
with no path ("GET http://host HTTP/1.1"). It now trims a leading "/"
instead; an empty name fails to read, so the request gets the index
page, as "/" already does.

The new test in internal/server sends such a request and expects 200
and the index page. export_test.go gives it a Server whose router has
only the web client route.

Model: opus-5-5

internal/server/routes_test.go

@@ -0,0 +1,41 @@
+package server_test
+
+import (
+	"io/fs"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"sneak.berlin/go/neoirc/internal/server"
+	"sneak.berlin/go/neoirc/web"
+)
+
+// An absolute-form request line with no path, such as
+// "GET http://example.com HTTP/1.1", leaves URL.Path empty.
+func TestSPAServesIndexForEmptyPath(t *testing.T) {
+	t.Parallel()
+
+	request := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "http://example.com", nil,
+	)
+	if request.URL.Path != "" {
+		t.Fatalf("URL.Path is %q, want it empty", request.URL.Path)
+	}
+
+	recorder := httptest.NewRecorder()
+
+	server.NewTestSPAServer().ServeHTTP(recorder, request)
+
+	if recorder.Code != http.StatusOK {
+		t.Fatalf("status %d, want %d", recorder.Code, http.StatusOK)
+	}
+
+	indexHTML, err := fs.ReadFile(web.Dist, "dist/index.html")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if recorder.Body.String() != string(indexHTML) {
+		t.Errorf("body is not the index page")
+	}
+}