feat: add Textual CSS stylesheet for TUI layout

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/ui/styles.tcss

@@ -0,0 +1,33 @@
+/* SneakyCode TUI Layout */
+
+Screen {
+    layout: vertical;
+}
+
+#chat-log {
+    height: 1fr;
+    border: none;
+    scrollbar-gutter: stable;
+}
+
+#streaming {
+    display: none;
+    height: auto;
+    max-height: 50%;
+    padding: 0 1;
+}
+
+#streaming.visible {
+    display: block;
+}
+
+/* StatusBar styles are in DEFAULT_CSS on the widget itself */
+
+Input {
+    dock: bottom;
+    margin: 0;
+}
+
+Header {
+    dock: top;
+}