feat: add global styles and CSS variables

src/styles/global.css

@@ -0,0 +1,36 @@
+*,
+*::before,
+*::after {
+  box-sizing: border-box;
+  margin: 0;
+  padding: 0;
+}
+
+:root {
+  --color-bg: #f1f5f9;
+  --color-surface: #ffffff;
+  --color-border: #e2e8f0;
+  --color-text: #1e293b;
+  --color-text-muted: #94a3b8;
+  --color-primary: #3b82f6;
+  --color-primary-hover: #2563eb;
+  --color-toolbar-bg: #ffffff;
+  --radius: 6px;
+  --shadow: 0 1px 3px rgba(0, 0, 0, 0.08);
+  --font-sans: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+  --font-serif: Georgia, 'Times New Roman', serif;
+}
+
+html, body {
+  height: 100%;
+  font-family: var(--font-sans);
+  background: var(--color-bg);
+  color: var(--color-text);
+  -webkit-font-smoothing: antialiased;
+}
+
+#app {
+  height: 100%;
+  display: flex;
+  flex-direction: column;
+}