[FEATURE] Export presets for YouTube, TikTok, Instagram with auto aspect ratios

[adamghaleb]

Description

The export dialog has quality presets (Low/Medium/High/Very High) but no platform-specific presets.

Expected Behavior

Add one-click export presets:

• YouTube: 1920x1080, H.264, 8Mbps, 30/60fps
• TikTok: 1080x1920 (9:16), H.264, 6Mbps, 30fps
• Instagram Reels: 1080x1920 (9:16), H.264, 5Mbps, 30fps
• Instagram Post: 1080x1080 (1:1), H.264, 5Mbps
• Twitter/X: 1280x720, H.264, 5Mbps

Each preset should auto-set resolution, bitrate, and codec.

Files

• `apps/web/src/components/editor/export-button.tsx`
• `apps/web/src/lib/export.ts`