feat: add global styles and CSS variables

src/styles/editor.css

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+.editor {
+  flex: 1;
+  display: flex;
+  justify-content: center;
+  padding: 2rem 1rem;
+  overflow-y: auto;
+}
+
+.editor .tiptap {
+  width: 100%;
+  max-width: 720px;
+  min-height: 60vh;
+  padding: 2rem;
+  background: var(--color-surface);
+  border-radius: var(--radius);
+  box-shadow: var(--shadow);
+  border: 1px solid var(--color-border);
+  outline: none;
+  font-family: var(--font-serif);
+  font-size: 1.05rem;
+  line-height: 1.7;
+  color: var(--color-text);
+}
+
+.editor .tiptap p {
+  margin-bottom: 0.75em;
+}
+
+.editor .tiptap h1,
+.editor .tiptap h2,
+.editor .tiptap h3 {
+  font-family: var(--font-sans);
+  font-weight: 700;
+  line-height: 1.3;
+  margin-top: 1.5em;
+  margin-bottom: 0.5em;
+}
+
+.editor .tiptap h1 {
+  font-size: 1.75rem;
+}
+
+.editor .tiptap h2 {
+  font-size: 1.4rem;
+}
+
+.editor .tiptap h3 {
+  font-size: 1.15rem;
+}
+
+.editor .tiptap p.is-editor-empty:first-child::before {
+  content: attr(data-placeholder);
+  float: left;
+  color: var(--color-text-muted);
+  pointer-events: none;
+  height: 0;
+}

src/styles/global.css

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+*,
+*::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;
+}