feat: export canvas as PNG or SVG image

[trmquang93]

Summary

• Adds Export as PNG and Export as SVG items to the File menu, exporting the canvas (entire flow, current scope, or rubber-band selection) for embedding in design docs, tickets, and chats.
• Selection-priority chain: multi-select ▸ scope root ▸ entire canvas.
• PNG renders at 2x pixel ratio (Retina) with a 16384px safety cap so large flows downscale gracefully instead of blanking out.
• SVG is self-contained (screen images embedded as data URLs) so it renders standalone in Figma/Illustrator/Notion.

Implementation

• New src/utils/exportCanvasImage.js — pure utility module with no React/JSX deps.
• Reuses the existing screenContentToHref pattern (handles imageData, svgContent, and wireframe content types via wireframeToSvg).
• Renders screens, bezier connections (color-coded by connectionPath), connection labels, sticky notes, and screen-group rectangles — matching the editor visuals while excluding editor chrome.
• Hotspots drawn as subtle dashed overlays so reviewers can see tap targets.
• No new runtime dependencies — uses native <canvas> and <svg> only.

Test plan

• Unit tests added: 19 new tests in exportCanvasImage.test.js covering selection filtering, bounding-box math, SVG output, color paths, XML escaping, and screen-content encoding.
• Full suite: 667/667 passed (npm test).
• Lint: clean (npm run lint).
• Build: clean, no new warnings (npm run build).
• Manual: try Export as PNG with no selection — expect entire canvas as PNG.
• Manual: rubber-band 2 screens, Export as SVG — expect only those screens + their direct connection.
• Manual: set a scope root, Export as PNG — expect only the scoped sub-flow.
• Manual: open the SVG in Figma — expect screen images visible, labels editable.

User guide

Updated src/pages/docs/userGuide.md with a new "Exporting Canvas Images (PNG / SVG)" section explaining scope rules, PNG-vs-SVG trade-offs, and what's included/excluded.

Closes backlog item 5.8.