[FEATURE] Add flowchart export to PNG/SVG/PDF

[rohitkumarnaidu]

Feature Description

Users can create flowcharts but cannot export them as images for presentations or documentation.

Proposed Solution

1. PNG Export using html2canvas
2. SVG Export generating clean SVG markup
3. PDF Export using jsPDF
4. Export dropdown menu with format options
5. Options for transparent background and high-DPI export

Why This Matters

• Export is a core feature for any diagramming tool
• Enables integration with documentation workflows
• Makes FlowCraft useful for academic and professional use

[rohitkumarnaidu]

Hey maintainer, I'd love to work on this issue and submit a PR. Could you please assign it to me?