feat: add export-as-image with format, scale, and background options

New ExportDialog component with scope (selection/all), filename,
format (PNG/JPEG/WebP), quality slider, scale multiplier, and
background color picker. Wired into Toolbar and Editor.

frontend/src/components/Toolbar.tsx

@@ -33,6 +33,7 @@ interface ToolbarProps {
   showLayers?: boolean;
   onToggleHelp?: () => void;
   onMmImport?: () => void;
+  onExport?: () => void;
   boardName?: string;
 }
 
@@ -148,6 +149,7 @@ export default function Toolbar({
   showLayers,
   onToggleHelp,
   onMmImport,
+  onExport,
 }: ToolbarProps) {
   const showStroke = activeTool === ToolType.PEN;
   const showFontSize = activeTool === ToolType.TEXT;
@@ -297,6 +299,16 @@ export default function Toolbar({
         </ActionBtn>
       )}
 
+      {/* Export */}
+      {onExport && (
+        <ActionBtn onClick={onExport} title="Export as image (Ctrl+Shift+E)">
+          <svg width="14" height="14" viewBox="0 0 14 14" fill="none" stroke="currentColor" strokeWidth="1.3">
+            <path d="M2 9v3h10V9" strokeLinecap="round" strokeLinejoin="round" />
+            <path d="M7 2v7M4 6l3 3 3-3" strokeLinecap="round" strokeLinejoin="round" />
+          </svg>
+        </ActionBtn>
+      )}
+
       {/* Spacer */}
       <div style={{ flex: 1 }} />