feat: set fixed A4 paper size for the editor canvas

src/lib/editor/Editor.svelte

@@ -32,5 +32,5 @@
 </script>
 
 <div class="editor">
-  <div bind:this={element}></div>
+  <div bind:this={element} class="paper"></div>
 </div>

src/styles/editor.css

@@ -6,14 +6,19 @@
   overflow-y: auto;
 }
 
-.editor .tiptap {
+/* Fixed A4 paper container (794px = 210mm at 96dpi) */
+.paper {
+  width: 794px;
+  max-width: calc(100vw - 2rem);
+  flex-shrink: 0;
+}
+
+.paper .tiptap {
   width: 100%;
-  max-width: 720px;
-  min-height: 60vh;
-  padding: 2rem;
+  min-height: 1123px; /* 297mm at 96dpi */
+  padding: 2.5rem 3rem;
   background: var(--color-surface);
-  border-radius: var(--radius);
-  box-shadow: var(--shadow);
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.12);
   border: 1px solid var(--color-border);
   outline: none;
   font-family: var(--font-serif);
@@ -22,13 +27,13 @@
   color: var(--color-text);
 }
 
-.editor .tiptap p {
+.paper .tiptap p {
   margin-bottom: 0.75em;
 }
 
-.editor .tiptap h1,
-.editor .tiptap h2,
-.editor .tiptap h3 {
+.paper .tiptap h1,
+.paper .tiptap h2,
+.paper .tiptap h3 {
   font-family: var(--font-sans);
   font-weight: 700;
   line-height: 1.3;
@@ -36,19 +41,19 @@
   margin-bottom: 0.5em;
 }
 
-.editor .tiptap h1 {
+.paper .tiptap h1 {
   font-size: 1.75rem;
 }
 
-.editor .tiptap h2 {
+.paper .tiptap h2 {
   font-size: 1.4rem;
 }
 
-.editor .tiptap h3 {
+.paper .tiptap h3 {
   font-size: 1.15rem;
 }
 
-.editor .tiptap p.is-editor-empty:first-child::before {
+.paper .tiptap p.is-editor-empty:first-child::before {
   content: attr(data-placeholder);
   float: left;
   color: var(--color-text-muted);