Add basic "join room" overlay

cove-tui/src/ui/layout.rs

@@ -0,0 +1,14 @@
+use tui::layout::Rect;
+
+pub fn centered(width: u16, height: u16, area: Rect) -> Rect {
+    let width = width.min(area.width);
+    let height = height.min(area.height);
+    let dx = (area.width - width) / 2;
+    let dy = (area.height - height) / 2;
+    Rect {
+        x: area.x + dx,
+        y: area.y + dy,
+        width,
+        height,
+    }
+}