feat: add global styles and CSS variables

src/styles/editor.css

@@ -0,0 +1,57 @@
+.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;
+}