feat: add njet.css with fixed-position dialog and window styles centered via transform

Introduce a new stylesheet defining `.njet-dialog` and `.njet-window` classes, both using fixed positioning, 50% centering via translate, 20px padding, and a 33px min-width.

src/snek/static/njet.css

@@ -0,0 +1,18 @@
+
+/* CSS version */
+.njet-dialog {
+  position: fixed;
+  top: 50%;
+  left: 50%;
+  transform: translate(-50%, -50%);
+  padding: 20px;
+  min-width: 33px;
+}
+.njet-window {
+  position: fixed;
+  top: 50%;
+  left: 50%;
+  transform: translate(-50%, -50%);
+  padding: 20px;
+  min-width: 33px;
+}