feat: add quality selector to PNG export (standard / high / ultra)

Clicking Export PNG now opens a modal with three quality presets:
- Standard (1× pixel ratio) — small file
- High (2×, default) — recommended for sharing
- Ultra (4×) — print quality

Adds ExportModal component, updates exportToPng() to accept a quality
param, and wires the modal into App.tsx replacing the direct export call.

frontend/src/utils/export.ts

@@ -1,14 +1,19 @@
 import { toPng } from 'html-to-image'
 
-/**
- * Export the React Flow canvas as a PNG and trigger a browser download.
- * Pass the `.react-flow` wrapper element.
- */
-export async function exportToPng(element: HTMLElement): Promise<void> {
+export type ExportQuality = 'standard' | 'high' | 'ultra'
+
+export const EXPORT_QUALITY_OPTIONS: { value: ExportQuality; label: string; pixelRatio: number; hint: string }[] = [
+  { value: 'standard', label: 'Standard', pixelRatio: 1, hint: '1× — small file' },
+  { value: 'high',     label: 'High',     pixelRatio: 2, hint: '2× — recommended' },
+  { value: 'ultra',    label: 'Ultra',    pixelRatio: 4, hint: '4× — print quality, large file' },
+]
+
+export async function exportToPng(element: HTMLElement, quality: ExportQuality = 'high'): Promise<void> {
+  const option = EXPORT_QUALITY_OPTIONS.find((o) => o.value === quality) ?? EXPORT_QUALITY_OPTIONS[1]
   const dataUrl = await toPng(element, {
     backgroundColor: '#0d1117',
+    pixelRatio: option.pixelRatio,
     style: {
-      // Exclude controls from the export
       '--xy-controls-display': 'none',
     } as Partial<CSSStyleDeclaration>,
   })