[Feature] Final Video Rendering & Export - Compose and Export the Complete Video

[vzeman]

Summary

Implement the final video compilation and export pipeline that assembles all approved scenes, applies global settings (resolution, format, quality), and produces the deliverable output file ready for distribution.

Requirements

• "Render Final Video" button that concatenates all approved scenes
• Rendering progress bar with time estimate
• Output format options: MP4 (H.264, H.265), MOV, WebM, GIF
• Resolution presets: 480p, 720p, 1080p, 4K, source resolution
• Frame rate options: 24, 25, 30, 60 FPS
• Quality presets: Draft (fast, ~2Mbps), Standard (~8Mbps), High (~16Mbps), Lossless
• Include/exclude audio in export
• Subtitle/caption export: SRT, VTT, burned-in captions
• Watermark overlay: add logo or text watermark (configurable position, opacity)
• Render queue: add multiple export configurations and render all
• Re-render specific scenes and update final video
• Social media export presets: YouTube (16:9), TikTok/Reels (9:16), Twitter/X (1:1)
• Export to local folder with auto-naming
• Export history with file size and duration
• Share sheet (macOS/iOS native share)

Acceptance Criteria

• Final render produces a single video file from all approved scenes
• All output format options (MP4, MOV, WebM, GIF) export correctly
• Quality presets apply correct bitrate settings
• Watermark overlay renders at configured position and opacity
• Social media presets produce correctly sized outputs
• Export history logs all renders with metadata

Tech Notes

• Use ffmpeg_kit_flutter for all encoding operations
• Store render presets in user preferences
• Implement render queue as a background isolate

Related

Part of the ai-video-editor Flutter application. Depends on #71 (Scene Continuation & Stitching). Integrates with #78 (Subtitle & Caption System).