feat: export interactive HTML prototype from flow

[trmquang93]

Summary

• Add Export Prototype to the File menu that generates a single self-contained .html file from the current flow
• The HTML prototype includes all screen images inlined as base64, invisible hotspot tap areas, a floating back button, sidebar screen list, conditional branch popups, and keyboard navigation
• Respects scope filtering — when viewing a sub-flow, only those screens are included in the export
• Includes 24 unit tests covering navigation resolution, scope filtering, sorting, title escaping, and edge cases
• User guide updated with full documentation of the prototype export feature

Changes

File	What
src/utils/generatePrototype.js	New — core prototype HTML generator with navigation map builder, screen serializer, CSS, and runtime JS engine
src/utils/generatePrototype.test.js	New — 24 Vitest tests
src/hooks/useImportExport.js	Wire onExportPrototype callback using generatePrototype + downloadPrototype
src/Drawd.jsx	Pass onExportPrototype, scopeScreenIds, and connectedFileName through to hooks/TopBar
src/components/TopBar.jsx	Add "Export Prototype" button to File dropdown menu
src/pages/docs/userGuide.md	Document the export prototype feature (how-to, behavior, scope filtering)

Test plan

• Open a flow with multiple screens and hotspots, click File > Export Prototype, verify a .html file downloads
• Open the exported HTML in a browser — tap hotspots to navigate between screens, verify back button works
• Test with conditional hotspots — verify the choice popup appears with labeled options
• Activate a scope root (sub-flow view), export prototype, verify only scoped screens are included
• Export with no screens — button should be disabled
• npm test passes (24 new tests + all existing)