UI infrastructure and simple "Help" dialog (#3)

Sets up some WIP infrastructure for building out the UI, and adds a simple popup
to show the navigation controls for the app.

studio-frontend/src/app/button.element.scss

@@ -0,0 +1,31 @@
+:host {
+	display: inline-flex;
+	align-items: center;
+	gap: 0.5rem;
+	padding: 10px 20px;
+	border-radius: 0.5rem;
+
+	font: {
+		family: "Fira Sans", sans-serif;
+		size: 15px;
+		weight: 500;
+	}
+	line-height: 1;
+
+	pointer-events: auto;
+	cursor: pointer;
+	user-select: none;
+
+	background: #0006;
+	color: #FFF;
+	backdrop-filter: blur(16px);
+	transition: background 0.1s linear;
+}
+
+:host(:hover) {
+	background: #0008;
+}
+
+.icon {
+	fill: currentColor;
+}