legal: close AGPL packaging gaps

Name the copyright holder, ship NOTICE with the image, serve the licence
from the panel footer on every page, and record the vendored htmx licence.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/legal/license_test.go

@@ -0,0 +1,21 @@
+package legal
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestEmbeddedLicenseMatchesRoot(t *testing.T) {
+	root, err := os.ReadFile(filepath.Join("..", "..", "LICENSE"))
+	if err != nil {
+		t.Fatalf("read root LICENSE: %v", err)
+	}
+	if !bytes.Equal(root, License) {
+		t.Fatal("internal/legal/LICENSE differs from the repository-root LICENSE; copy the root file over")
+	}
+	if len(License) == 0 {
+		t.Fatal("embedded LICENSE is empty")
+	}
+}