tweak(ui): default export border width to 2

Remove the `/export-png` endpoint and hide the preview section in the UI when exporting as SVG.

feat(export): add live preview and configurable export options

feat: explicitly set PNG compression level to 0 for stitched image encoding.

feat: Add horizontal PNG preview and export functionality for selected frames with optional separators.

feat: update film strip background color to light purple.

feat: enhance filmstrip SVG generation with transparent sprocket holes and improved layout

feat: add batch export functionality for multiple video files as SVG ZIP

feat: enhance filmstrip SVG generation with uniform outer margins and canvas background

feat: enhance filmstrip SVG generation with realistic 35mm styling and frame labels

feat: export selected frames as filmstrip-style embedded SVG

feat: add minimal web video frame extractor with uniform sampling